المناظير الحديثة كاميرات تسبح في الجسم
شهدت السنوات الاخيرة ثورة كبرى في مجال الجراحة تمثلت في ابتكار تقنيات واجهزة دقيقة بعضها متناه في الصغر لمساعدة الاطباء على تشخيص الامراض واجراء العمليات الجراحية بطرق حديثة تقلل نسبة المخاطر بشكل اكبر بكثير مما هو الحال عند اجراء العمليات الجراحية التقليدية، خاصة الكبرى منها. وعلى نحو مغاير مما نعرفه عن المناظير التقليدية التي ما زال الاطباء في مختلف انحاء العالم يستخدمونها حتى اليوم، فإن المناظير الجديدة هي كاميرات لا يزيد حجمها عن حجم حبة دواء عادية، لكنها بعد ان تقوم بانجاز المهمة المطلوبة منها فانها تنهيها بالخروج من الجسم ومن ثم استخراج ما انتجته من صور خلال رحلتها من البلعوم حتى الشرج في مدة ثماني ساعات داخل الجسم.
من آخر الابتكارات التي طورها علماء أمريكيون أصغر جهاز منظار داخلي من شأنه أن يحدث ثورة طبية على صعيد تشخيص الأمراض وعلاجها.
وهذه التقنية الجديدة تعد أحد أصغر الأجهزة في العالم القادر على سبر أغوار جسم الإنسان، وتتمثل في نظام “سكانر” ليزري بحجم كبسولة الدواء وموصول بألياف بصرية ويستعمل لكشف وتصوير سرطان المريء.
وما يميز الجهاز الجديد الذي ابتكره مهندسون في جامعة واشنطن هو مرونته وحجمه الذي لا يتجاوز نصف حجم أقرب منافسيه، فضلا عن سهولة استخدامه مقارنة بالمنظار العادي الذي يزعج المرضى والذي من الممكن أن يعلق في الفم أثناء إقحامه بشدة عبر الفم حتى يصل إلى المكان المطلوب داخل الجسم.
غير ان مخترعي التقنية الجديدة يواجهون تحدي إدراج نظامهم الجديد في قائمة “الضمان الصحي” الامريكية، حيث كان من المتوقع أن 30 مليون أمريكي سيكونون مستعدين لدفع ثمن الفحص بهذه الكاميرا العجيبة متناهية الصغر والتي لا تتطلب منهم سوى بلعها ليرى الطبيب ما في داخل أحشائهم ثم الخروج مع البراز لاحقاً.
وعلى الرغم من ذلك يأمل المخترعون من جامعة واشنطن أن يتمكنوا من إدراج الكاميرا الكبسولة الجديدة، في قوائم شركات التأمين على اعتبار أن الفحص بهذه الطريقة سيكون أرخص ثمنا وأسرع من الكبسولة السابقة بالإضافة إلى أنها تتيح للطبيب حرية أكبر في التحكم بها.
وعلى النقيض من الكبسولة المنظار السابقة التي كان المريض يبلعها لتخرج من الجسم مع الفضلات، فإن الكبسولة الليزرية الجديدة التي طورتها جامعة “واشنطن” يمكن أن يسحبها الطبيب مثل المنظار التقليدي وهي مجهزة بضوء ملحق بنهايتها وتستخدم لفحص تجويف الجسم وأعضاء مختلفة منه.
والميزة المهمة الأخرى التي تتمتع بها هذه الكبسولة هي أنها لا تتطلب تخدير المريض كما يجري لدى استخدام أنواع المناظير العادية.
ويعتقد فريق جامعة واشنطن، أن الفائدة الأولى التي سيحصل عليها الطب من هذه الكبسولة تتمثل في علاج المرضى الجدد الذي ينفرون من طريقة تشخيص سرطان المريء لأنها غير مريحة على الإطلاق بالنسبة لهم علاوة على ارتفاع تكلفتها والوقت الطويل الذي تستغرقه.
وقال الباحث إيريك شيبل، أستاذ الهندسة الميكانيكية بجامعة واشنطن: “ستسهم هذه الكبسولة إلى حد كبير في دفع عدد أكبر من الناس للخضوع للاختبارات الكشف”.والجدير بالذكر أن آلاف الأشخاص يصابون سنويا بسرطان المريء.
وقد شهدت أمريكا وحدها السنة الماضية، أكثر من 15 ألف حالة إصابة، ووفاة 14 ألف مريض، حسب تقارير المعهد الأمريكي للسرطان.
وغالبا ما ينصح الأطباء الناس بالابتعاد عن تناول الكحول والتدخين كي ينأوا بأنفسهم عن شبح هذا المرض الخطير.
ومن العوامل الأخرى التي تساهم في الإصابة بالمرض حالة الارتجاع المعوي والمتمثل بعودة الطعام المهضوم أو مادة الصفراء التي يفرزها الكبد إلى المريء مرة أخرى ويمكن أن تؤدي حينذاك إلى تهيج المريء وظهور خلايا غير طبيعية تزيد من احتمالية الإصابة بالسرطان.
وكبسولة جامعة “واشنطن” قادرة على التقاط صور فيديو أثناء حركة جهاز نظام “السكانر” إلى الأعلى أو الأسفل داخل أنبوب المريء. ومثل هذا التحكم بالكاميرا كان غير ممكنا في الكبسولة السابقة نظرا لضيق نطاقها.
وتأتي التقنية المتطورة لجامعة واشنطن على شكل كبسولة بلاستيكية كي يكون وزنها خفيفا وتسهل عملية بلعها.
وعلى أي حال، فانه إذا ما اكتشفت الكبسولة المنظار أي خلايا غير طبيعية فإنه ينبغي على المريض لاحقا الخضوع لإجراء منفصل يتم بموجبه أخذ خزعة من هذا الورم لفحصه مختبريا.
وقالت ليزا نورتون، مدير التقنية في وحدة “تيك ترانسفير” التابعة لجامعة واشنطن والتي تساعد على ترخيص اختراعات الجامعة، إنه من المتوقع أن توقع الجامعة قريبا اتفاقا مع إحدى الشركات المصنعة للأجهزة الطبية، من أجل البدء بمرحلة إنتاج هذه الكبسولة. وفي حالة حصول الاختراع على موافقة (إدارة الغذاء والدواء) الأمريكية يمكن حينذاك طرحه للبيع في الأسواق في غضون سنوات قليلة. ويعتزم إيريك شيبل وزملاؤه في المضي قدماً في التفكير باستخدامات أخرى يمكن أن تدخل الكبسولة المرنة فيها ومن دمجها مع أجهزة تساعد على استكشاف مناطق عصية وتصوير البنكرياس أو قناة فالوب. كما يمكن استعمال مثل هذا الاختراع في مجالات أخرى مثل تصوير الزوايا البعيدة في أجنحة الطائرة. وإذا ما نجحت عملية التسويق فان جامعة واشنطن ستنال ثلثي العائد، في حين يذهب الثلث الآخر إلى إيريك شيبل وزملائه أو المشاركين في الاختراع وعددهم 15. وتشهد العديد من المستشفيات المتطورة في العالم إقبالاً متزايداً على تقنية 2004PillCam ESO) وهي أيضاً بحجم الكبسولة ومزودة بكاميرتين في كل طرف يستخدمها الأطباء لتشخيص أمراض المريء في الحالات التي يمكن أن تتطور إلى سرطان.
وهذه الكبسولة قادرة على التقاط ما يقارب 2600 صورة للمريء ( 14 في كل ثانية)، وتنتقل الصور من الكبسولة إلى جهاز تسجيل وفي أقل من 20 دقيقة يكون أمام الطبيب المعالج عدد من الصور كافٍ لتشخيص الحالة بدقة متناهية. على أي حال.
وهناك إجراءات بسيطة ينبغي أن يتبعها المريض قبل خضوعه للفحص بهذه الكبسولة وهي :
التوقف عن الأكل والشرب قبل ساعتين من بلع الكبسولة.
يستلقي المريض على ظهره ويبلع الكبسولة مع الماء.
بعد بلعه للكبسولة يرفع الطبيب المريض بمقدار 30 درجة كل دقيقتين وخلال فترة ست دقائق من البلع إلى أن يجلس بصورة مستقيمة.
تنزل الكبسولة داخل المريء خلال حوالي 3 دقائق.
تنتقل الصور إلى جهاز تسجيل مثبت على حزام يضعه المريض حول خصره. وتستغرق كل عملية فحص حوالي 20 دقيقة، وتستخدم الكبسولة لمرة واحدة وتخرج لاحقا من الفتحة المعوية بصورة طبيعية ومن دون أي ألم وذلك خلال 24 إلى 72 ساعة.
ويقول الخبراء إن هناك عدة فئات من البشر ينبغي ألا تستخدم الكبسولة بينها من يعانون من مشاكل في البلع أو يعانون أو يشك في أنهم يعانون من عوائق، أو تضيق في المريء. كما أنها محظورة على المرضى الذين يحملون منظم ضربات قلب أو أي أجهزة كهربائية أخرى مزروعة داخل الجسم.
الفرق بين المناظير العادية ومنظار الكبسولة:
تتطلب عمليات المناظير الآتي:
امتناع المريض عن الطعام والشراب لمدة لا تقل عن 8 ساعات.
تعاطي أدوية مسكنة قبل إدخال المناظير في الجسم.
عملية لإدخال المنظار في الجسم.
البقاء في المستشفى بضع ساعات حتى يزول مفعول المسكنات قبل الخروج.
حاجة المريض لمرافق يصطحبه عند خروجه بعد الانتهاء من العملية، فالمريض لن يستطيع قيادة السيارة، بل وليس مخولاً بذلك طوال ذلك اليوم فتأثير المسكن يبقى في الدم لساعات طويلة.
أما كبسولة المريء فلا تتطلب أيا من ذلك، إذ إن كل ما هو مطلوب هو:
أن يمتنع المريض عن الطعام والشراب لمدة ساعتين فقط قبل ابتلاع الحبة “كبسولة الكاميرا”
يطلب من المريض الاستلقاء على ظهره في وضع أفقي اثناء ابتلاع الكبسولة لمدة دقيقتين يرافق ذلك وضع 3 أجهزة استشعار فوق صدره وحزام تتدلى منه مسجلة رقمية ومن ثم يطلب من المريض النهوض قليلاً ثم الجلوس ثم تنفصل الأجهزة عنه وكل ذلك يستغرق عشرين دقيقة فقط.
آلية عمل الكبسولة
تقوم كبسولة الأمعاء بالتقاط صورها بعد بلعها مباشرة وأثناء مرورها الطبيعي مع حركة الأمعاء، والتي تلتقطها أجهزة الاستشعار الثمانية المثبتة ببطن المريض لترسلها إلى جهاز التسجيل المثبت على حزام الخاصرة، وفي هذه الأثناء يقوم المريض بممارسة حياته الطبيعية لمدة 8 ساعات، وبعد فصل الاجهزة عن المريض يقوم الطبيب بقراءة شريط الفيديو المسجل بجهاز التسجيل عن طريق الكمبيوتر.

Bragg Mirrors

Definition: mirror structures based on Bragg reflection at a period structure

A Bragg mirror (also called distributed Bragg reflector) is a structure which consists of an alternating sequence of layers of two different optical materials. The most frequently used design is that of a quarter-wave mirror, where each optical layer thickness corresponding to one quarter of the wavelength for which the mirror is designed. The latter condition holds for normal incidence; if the mirror is designed for larger angles of incidence, accordingly thicker layers are needed.

The principle of operation can be understood as follows. Each interface between the two materials contributes a Fresnel reflection. For the design wavelength, the optical path length difference between reflections from subsequent interfaces is half the wavelength; in addition, the reflection coefficients for the interfaces have alternating signs. Therefore, all reflected components from the interfaces interfere constructively, which results in a strong reflection. The reflectivity achieved is determined by the number of layer pairs and by the refractive indexbandwidth is determined mainly by the index contrast. contrast between the layer materials. The reflection

Figure 1 shows the field penetration into a Bragg mirror made of eight layer pairs of TiO₂ and SiO₂. The blue curve shows the intensity distribution of a wave with the design wavelength of 1000 nm, incident from the right-hand side. Note that the intensity is oscillating outside the mirror due to the interference of the counterpropagating waves. The gray curve shows the intensity distribution for 800 nm, where a significant part of the light can get through the mirror coating.

Figure 1: Field penetration into a Bragg mirror.

Figure 2 shows the reflectivity and the group delay dispersion as functions of the wavelength. The reflectivity is high over some optical bandwidth, which depends on the refractive index contrast of the materials used and on the number of layer pairs. The dispersion is calculated from the second derivative of the reflection phase with respect to the optical frequency. It is small near the center of the reflection band, but grows rapidly near the edges.

Figure 2: Reflectivity (black curve) and chromatic dispersion (blue curve) of the same mirror as above.

Figure 3 shows with a color scale how the optical field penetrates into the mirror. It can be seen that there is little field penetration well within the reflection band.

Figure 3: Field penetration into the Bragg mirror as a function of wavelength. The colors indicate the optical intensity inside the mirror.

Types of Bragg Mirrors

Bragg mirrors can be fabricated with different technologies:

• Dielectric mirrors based on thin-film coating technology, fabricated for example with electron beam evaporation or with ion beam sputtering, are used as laser mirrors in solid-state bulk lasers.
• Fiber Bragg gratings, including long-period fiber gratings, are often used in fiber lasers and other fiberultraviolet light. Similarly, volume Bragg gratings can be made in photosensitive bulk glass. devices. They can be fabricated by irradiating a fiber with spatially patterned
• There are various types of Bragg reflectors used in other waveguides, based on, e.g., corrugated waveguide structures which can be fabricated via lithography. Such kind of gratings are used in some distributed Bragg reflector or distributed feedback laser diodes.

There are other multilayer mirror designs which deviate from the simple quarter-wave design. They generally have a lower reflectivity for the same number of layers, but can be optimized e.g. as dichroic mirrors or as chirped mirrors for dispersion compensation.

Diode Bars

Definition: a type of semiconductor laser containing a one-dimensional array of broad-area emitters

Diode bars are high-power semiconductor lasers (laser diodes), containing a one-dimensional array of broad-area emitters. They typically contain between 20 and 50 emitters, each being e.g. 100 μm wide. A typical commercial device has a laser resonator length of the order of 1 mm, is 10 mm wide and generates tens of watts of output power; some prototypes even reach hundreds of watts (possibly with a reduced lifetime). Within the last 20 years, the price per watt has come down by nearly two orders of magnitude, while at the same time the brightness has been very significantly improved.

Figure 1: Schematic view of the semiconductor chip of a diode laser bar with a fill factor of 50%.

Most diode bars operate in the wavelength region from 780 to 860 nm or between 940 and 980 nm, with the wavelengths of 808 nm (for pumping neodymium lasers) and 940 nm (for pumping Yb:YAG) being most prominent. Another important wavelength is 975–980 nm for pumping erbium-doped or ytterbium-dopedhigh-power fiber lasers and amplifiers.

Figure 2: Photograph of actively cooled diode bars from JENOPTIK Laser Diode GmbH.

A non-ideal property of diode bars is the spatial pattern of its emission. In the “fast axis” direction (corresponding to the vertical direction in Figure 1 and 2), the emission comes from a very narrow region, so that the beam quality is not far from diffraction-limited (M² factor not far above 1) despite the strong beam divergence angle of typically 30–40° FWHM (full width at half-maximum). On the other hand, the “slow axis” direction has a very wide emitting region, so that despite the much smaller divergence angle (of the order of 6–10°) the beam quality is very poor, with the M² factor > 1000. Significant efforts are therefore often required for conditioning the output of a diode bar (or of multiple diode bars in the form of a diode stack).

The large numerical aperture (NA) of typically ∼ 0.6 for the fast axis requires high-NA aspherical lenses for collimating the beams while preserving the beam quality. Microoptic fast axis collimators, containing an array of aspherical cylindrical lenses, are often used for that purpose. There are also microoptic modules which serve as slow axis collimators. The demands for slow axis collimation are lower concerning the NA of typically ∼ 0.1, but nevertheless not uncritical for diode bars with high fill factor (see below). When beams with approximately circular beam waist and reasonable beam quality are required, a special beam shaper for symmetrization of the beam quality can be applied.

A potentially disturbing property of diode bars is the “smile” – a slight bend of the horizontal line connecting the emitters. Smile errors can have detrimental effects on the ability to focus beams from diode bars. There are some advanced beam conditioning schemes where the smile of individual diode bars is compensated.

The reason for the use of diode arrays instead of simply making very broad single emitters is that the latter would suffer from amplified spontaneous emission or parasitic lasing in the transverse direction, or from the formation of filaments. Diode arrays can be operated with a more stable mode profile, consisting of one so-called beamlet from each emitter. There are several techniques which exploit some degree of coherent coupling of neighbored emitters, leading to better beam quality. Such techniques include those directly applied in the fabrication of the diode bars and others involving external cavities. Most diode bars, however, are used without such a technique.

Note that the array geometry makes diode bars very suitable for methods of coherent or spectral beam combining, which make it possible to obtain a much higher beam quality.

Fiber Coupling

Many diode bars are sold in fiber-coupled form, because this often makes it much easier to utilize their output and also makes it possible to mount the diode bars with their cooling arrangement in some distance from the place where the light is used (e.g. a diode-pumped laser head). Usually, the light is coupled into a single multimode fiber, using either a simple fast-axis collimator and no beam conditioning in the slow-axis direction, or a more complex beam shaper to preserve the brightness better. It is also possible to launch the beamlets from the emitters into a fiber bundle (with one fiber per emitter).

Cooling

The semiconductor chip of a diode bar is soldered to a thin submount, which also gives one of the electrical connections. An insulated wire bond plate provides the second connection. The submount is then mounted on some heat sink, which is often water-cooled (with a macrochannel or microchannel cooler), allowing for a high fill factor (ratio of emitter width to total width of emitting region) of e.g. 80% and thus for a high brightness. Conduction-cooled bars (often used with a thermoelectric cooler) have a lower fill factor of e.g. 30%, because the heat can be extracted less efficiently. In both cases, diode bars are often produced in sealed packages.

Power Efficiency

Electrically, the different emitters are all connected in parallel. This means that the overall drive current is substantial, of the order of tens or even hundreds of amperes. One obtains roughly 1 W of optical output power per 1 A of current; combined with the typical voltage drop of ∼2 V, this results in a power efficiency (→ wall-plug efficiency) of the order of 50%. Further developments are under way (e.g. in the context of the SHEDS program of DARPA) to reach 80%. Crucial technological issues are to reduce the operating voltage by optimizing electrical contacts and layer structures, to reduce further the thermal impedance, and to improve methods for facet passivation, effectively allowing higher optical intensities without the risk of catastrophic damage. Improved power efficiency reduces the total electrical power demands and also the demands on the cooling system, and further usually allows for higher brightness. Often it comes with the additional effect of reducing the price per watt of output power, which is already well below 30 US dollars.

Lifetime

Under ideal conditions, diode bars can have lifetimes of many thousands of hours. However, devices often fail long before the specified lifetime is reached. This is not necessarily a consequence of faults in the production or the design, but can be caused by a variety of factors outside the control of the manufacturer, such as short voltage spikes caused by a defective or ill-designed diode laser driver (often during switching the device on or off), or by too high an operation temperature, which itself can be caused by too high a drive current or insufficient cooling. Water cooling is usually fairly effective, but its effectiveness can be strongly compromised by corrosion, which can occur e.g. when the specifications for the chemical condition of the cooling water (particularly its ion content) are not met. On the other hand, too strong cooling can cause problems via condensation, when the dew point is reached. Obviously, the whole diode laser system has to be properly designed in order to exploit the full lifetime potential of the bars.

Emission Bandwidth

The emission bandwidth of a diode bar typically amounts to several nanometers, which can be more than desirable for pumping a solid-state laser. In addition, there is typically a tolerance of several nanometers for the center wavelength. A strongly reduced wavelength tolerance and emission bandwidth may be achieved with optical feedback, e.g. from a volume Bragg grating. This can facilitate, e.g., the pumping of a solid-state bulk laser. Bandwidth and exact center wavelength can also be important for spectral beam combining.

Applications

High-power diode bars are used e.g. in material processing (e.g. welding and certain surface treatments), in medical applications (e.g. photodynamic therapy, tattoo removal, laser surgery), or for pumping high-power solid-state lasers (bulk or fiber lasers). Diode bars are also developed further for military use as battlefield laser weapons. In the future, they may also increasingly be applied in large-volume consumer products such as cars.

For very high powers (above roughly 100 W), one uses diode stacks, which are essential several diode bars stacked in the vertical direction.

Brewster Windows

Definition: transparent plates which are oriented at Brewster's angle

There are situations where an optical beam must be sent through some transparent window, whereas the optical losses occurring at this window must be very small. A typical example is a helium–neon laser with a sealed glass tube and external resonator mirrors as shown in Figure 1, where glass windows separate the laser gas mixture from ambient air. Given the small gain and the small output coupler transmission, the losses at these interfaces need to be far below 1% per pass. This is achieved by using Brewster windows, where the angle of incidence is close to Brewster's angle. In that situation, the reflectivity at the air–glass interfaces becomes very small for p-polarized light, i.e., when the polarization direction is in the plane of incidence.

Figure 1: Setup of a helium–neon laser. Although two glass windows are within the laser resonator, these induce very little optical loss for p-polarized light, as they are oriented for Brewster-angled incidence.

An uncoated glass plate at normal incidence would normally have a reflectivity of several percent on each side. With an anti-reflection coating, this could be reduced to e.g. 0.2%. Brewster windows can have at least 10 times lower losses. In addition, any residual reflection will leave the resonator, rather than lead to interference effects (as can occur for windows with normal incidence). Of course, both Brewster windows in a setup as shown above must have exactly the same orientation.

Due to the significant loss difference between p and s polarization, the polarization of laser emission is usually forced to be in the p direction. In many lasers, this is the only effect determining the polarization direction.

A potential disadvantage of Brewster windows (or other polarizing optical elements) in a laser resonator is that large depolarization loss can arise if, e.g., thermal effects within a laser crystal affect th

Thin-disk Lasers

Definition: solid-state bulk lasers having a very thin disk of laser-active material as the gain medium

The thin-disk laser (sometimes called thin-disc laser or active-mirror laser) is a special kind of diode-pumpedhigh-power solid-state laser, which was introduced in the 1990s by the group of Adolf Giesen at the University of Stuttgart, Germany. The main difference from conventional rod lasers or slab lasers is the geometry of the gain medium: the laser crystal is a thin disk, where the thickness is considerably smaller than the laser beam diameter, so that the heat generated is extracted dominantly through one end face, i.e., in the longitudinal rather than in the transverse direction. The cooled end face has a dielectric coating which reflects both the laser radiation and the pump radiation.

Figure 1: Setup of an 8-kW laser based on four thin-disk laser heads (seen on the left side). The photograph was kindly provided by TRUMPF.

The thin disk is also often called an active mirror [1], because it acts as a mirror with laser gain. Within the laser resonator, it can act as an end mirror or as a folding mirror. In the latter case, there are two double passes of the laser radiation per resonator round trip, so that the gain per round trip is doubled and the threshold pump power is reduced.

Figure 2: Schematic setup of a thin disk laser head. The pump optics (not shown) are arranged for multiple passes of the pump radiation. The heat is extracted in the longitudinal direction, which minimizes thermal lensing effects.

The thin-disk laser should not be confused with the rotary disk laser, where the gain medium is a quickly rotating disk, which is usually a few millimeters thick.

Reduced Thermal Issues at High Output Powers; Power Scalability

Due to the small thickness of the disk (e.g. 100–200 μm for Yb:YAG), the temperature rise associated with the dissipated power is small. In addition, the temperature gradients are dominantly in a direction perpendicular to the disk surface and thus cause only weak thermal lensing and depolarization loss. This allows for operation with very high beam quality due to the weak thermal beam distortions, and stable operation can be achieved over a wide range of pump powers.

A very important property arising from the thin-disk geometry is power scalability in a strict and meaningful sense. The scaling procedure is simple: for example, the output power can be doubled by applying twice the pump power to twice the area on the disk, while keeping the disk thickness and doping level constant. The laser resonator has to be modified so as to double the mode area in the disk. With this scaling procedure applied, the new design with twice the output power has unchanged peak optical intensities and a nearly unchanged maximum temperature in the disk (the latter basically because the cooling area has also been doubled). As far as thermal lensing results from the temperature dependence of the refractive index, the dioptric power (inverse focal length) of the thermal lens is reduced to half of the original value, which just compensates the doubled sensitivity of the larger mode to focal length changes. The power has thus been scaled without increasing optical intensities, the magnitude of temperature rises, or thermal lens problems. Strictly, thermal lensing effects can be increased by the power scaling as far as they result from mechanical stress in the disk, but this problem can be kept small by keeping the disk thickness small. Another limitation arises from amplified spontaneous emission (ASE) in the transverse direction, which ultimately limits the gain achievable in the longitudinal direction, but becomes severe only at very high power levels with many kilowatts from a single disk. The use of a composite disk with an undoped part on top of the doped part can also strongly suppress ASE; at least for continuous-wave lasers, the ASE limit may be pushed to the order of 1 MW from a single disk.

Power scalability in a wide range is achieved even for passively mode-locked thin-disk lasers (see below). Here, the doubling of the output power also involves doubling of the mode area on the SESAM, so that optical intensities in that device and also cooling issues are – contrary to naïve expectations – not limiting factors. Limitations rather arise from the problem of doing dispersion compensation at high power levels.

So far, hundreds of watts of output power in a diffraction-limited continuous-wave beam have been obtained. By using multiple thin-disk laser heads within one laser resonator, several kilowatts of power can be generated; thin-disk lasers with 8 kW output power in a multimode beam are commercially available, and similar or even higher power levels in nearly diffraction-limited beam will be realized in the near future. With mode-locked thin-disk lasers, ∼ 80 W of average output power has been achieved.

Multipass Pumping

The small thickness of the disk typically leads to inefficient pump absorption when only a single or double pass is used. This problem is normally solved by using a multi-pass pump arrangement, which can be made fairly compact when using a well-designed optical setup, typically containing a parabolic mirror and prism retroreflectors. Such arrangements easily allow one to arrange for e.g. 8 or 16 double passes of the pump radiation through the disk without excessively stringent requirements on the pump beam quality.

The pump source of a thin-disk laser is usually based on high-power diode bars, either in fiber-coupled form or with free-space power delivery. A typical pump wavelength is 940 nm for Yb:YAG, whereas ytterbium-doped tungstate crystals can be more efficiently pumped near 981 nm.

Thin-Disk Gain Media

The most often used gain medium for thin-disk lasers is Yb:YAG. Compared with Nd:YAG, it has a shorter emission wavelength (typically 1030 nm), a smaller quantum defect (reducing the dissipated power), a longer upper-state lifetime (improving energy storage for Q switching), and a larger gain bandwidth (e.g. for shorter pulses with mode locking). On the other hand, it is a quasi-three-level gain medium with significant reabsorption at the laser wavelength, and thus requires higher pump intensities. The thin-disk principle is well adapted to these parameters.

For broad wavelength tuning and for ultrashort pulse generation, other ytterbium-doped gain media offer a still wider gain bandwidth. Examples are tungstate crystals (Yb:KGW, Yb:KYW, Yb:KLuW), Yb:LaSc₃(BO₃)₄₄ (Yb:CALGO) and Yb:YVO₄. (Yb:LSB), Yb:CaGdAlO

Nd:YAG or Nd:YVO₄ may also be used in thin-disk lasers, e.g. when a wavelength of 1064 nm is required, or when the much smaller saturation energy of Nd:YVO₄ is relevant.

Spatial Hole Burning

An interesting consequence of the small disk thickness is that spatial hole burning usually cannot be avoided, even if a thin-disk laser is built with a ring resonator. (Note that due to the small ratio of thickness and beam radius, counterpropagating waves in the disk always have a strong overlap, so that an interference pattern is generated even in a ring resonator.) Nevertheless, single-frequency operation is possible by using an appropriate wavelength filter (etalon) in the resonator. For passive mode locking (see below), spatial hole burning in the thin disk distorts the shape of the gain spectrum, causing a variety of instabilities, but also allowing for significantly shorter pulses in the optimum range of parameters [5].

Q-switched Pulse Generation

Thin-disk lasers are well suited for generating high-energy nanosecond pulses with high beam quality, as required for, e.g., some kinds of laser material processing. The typically used gain medium Yb:YAG offers a significantly better energy storage (longer upper-state lifetime) compared with, e.g., Nd:YAG. A somewhat limiting factor, however, is the moderate gain (compared with that of an end-pumped bulk laser), which makes it difficult to achieve very short (few-nanosecond) pulses.

Mode-locked High-power Thin Disk Lasers

Thin-disk lasers are particularly attractive for the generation of ultrashort pulses at very high power levels. In addition to the high-power capability, the main advantages in this context are

• the ease of achieving diffraction-limited operation (which is a prerequisite for mode locking)
• the broad gain bandwidth of Yb:YAG (the so far most suitable gain medium for thin-disk lasers)
• the small nonlinearity of a thin disk, which helps to avoid excessive nonlinear phase shifts despite the high intracavity peak intensities

One of the initial challenges was to find a suitable mode-locking mechanism. Even though originally it was widely believed that passive mode locking with semiconductor saturable absorber mirrors (SESAMs) would not be possible at very high power levels – at least not without first developing special high-power SESAMs, possibly based on improved semiconductor materials –, the author's research group at ETH Zürich demonstrated around 2000 that both thermal and non-thermal issues can be relatively easily managed even at very high power levels, if only the design parameters of the overall laser system (and not only of the SESAM) are properly chosen. In other words, SESAM damage then does not constitute a limiting factor for the power scaling of mode-locked thin-disk lasers; in fact, it was found that the mode-locked thin-disk laser is the first truly power-scalable femtosecond laser. However, the design of such lasers involves a number of subtle issues, related to e.g. spatial hole burningdispersion compensation, and a trial-and-error approach not based on a solid understanding of various details is prone to fail, e.g. by not managing to suppress certain types of instabilities. and to

So far, thin-disk lasers have lead to the highest average output power of 80 W from a mode-locked laser [8, 10], and pulse energies of > 10 μJ combined with sub-picosecond pulse durations are possible [19, 20]. While typical pulse durations with Yb:YAG are around 700–800 fs, significantly shorter pulses are possible, e.g. with ytterbium-doped tungstate crystals such as Yb:KGW or Yb:KYW [7].

Amplifiers for High Pulse Energies

Thin-disk laser heads can also be used for regenerative amplifiers [11]. The relatively small gain of the thin disk can be compensated with a larger number of resonator round trips, even though this makes the amplifier more sensitive to optical losses and nonlinearities. Therefore, it can be advantageous to arrange for multiple passes of the signal radiation through the disk in each resonator round trip.

It is also possible to construct a purely multipass amplifier (without a resonator and optical switch), but this approach limits the overall gain and requires a carefully optimized setup in order to preserve a high beam quality.

Nonlinear Frequency Conversion

High-power continuous-wave green lasers can be easily realized in the form of intracavity frequency-doubledoptical parametric oscillators, amplifiers and generators [6]. A high-power RGB source based on a mode-locked thin-disk laser has also been demonstrated [10]. The high peak power of mode-locked thin-disk lasers allows for efficient nonlinear frequency conversion with critical phase matching in LBO crystals, thus not requiring crystal ovens for most or all conversion stages. thin-disk lasers. For Q-switched or mode-locked lasers, extracavity doubling is often more practical. Thin-disk lasers are also very interesting pump sources for

Further Development of the Thin-Disk Concept

The thin-disk laser concept allows for further variations. For example, side pumping of the disk may allow for even higher output powers while reducing the requirements on the pump beam quality. This approach, developed at the Lawrence Livermore National Laboratory, is based on a composite laser crystal [22]. An undoped YAG disk, which is bonded to a Yb:YAG disk, brings various benefits: It reduces the beam quality requirements for the pump source, reduces the tendency for transverse amplified spontaneous emission (ASE) and parasitic lasing in large disks, increases the mechanical strength and may improve the cooling. The side-pumped concept may thus allow scaling to much higher powers, even though not necessarily with diffraction-limited beam quality. An interesting option is to use a composite ceramic gain medium which is ytterbium doped only in the center region and not in the outside regions, which are used only to deliver the pump power.

It is also interesting to develop cryogenic thin-disk lasers, as cryogenic cooling greatly reduces thermal effects at high power levels.

Competition with Fiber Lasers

Thin-disk lasers are currently facing fierce competition from high-power fiber lasers and amplifiers. In continuous-wave operation, these can currently deliver even higher powers in close to diffraction-limited beams. However, they are generally considered to be still less mature for industrial applications. Within the next few years, both thin-disk lasers and fiber lasers are expected to show significant further progress, and it is currently not clear which technology will acquire the larger market share. See also the article on fiber lasers versus bulk lasers.

In the domain of ultrashort pulse generation, fiber amplifier systems allow one to reach even higher average powers and shorter pulse durations than thin-disk lasers can generate without amplification. However, thin-disk lasers will probably maintain superiority for the generation of pulses with high energies, particularly when high pulse quality (concerning temporal shape, low chirp, and stable linear polarization) is required. A key issue in this context is that both the small disk thickness and the larger beam diameter on the disk lead to a nonlinearity which is much smaller than that in a fiber laser resonator.

A detailed comparison of thin-disk versus fiber lasers is complex and has to take into account many aspects which depend on the particular application. For example, issues such as emission bandwidth, pulse quality and stability of polarization state can be essential in some cases but insignificant in others.

Semiconductor Disk Lasers

The thin-disk geometry is also used in vertical external cavity surface-emitting lasers (VECSELs), a kind of semiconductor lasers. In that case, multiple passes of the pump are usually not required due to the strong absorption of semiconductor materials. However, the concept with multiple pump passes has recently also been applied to such semiconductor lasers, where it allows for a reduced quantum defect and thus for reduced heating and potentially higher powers [12]. To date it is not clear whether this will lead to more efficient and practical lasers; tens of watts of output power have so far been reached with the original concept, not requiring multiple pump passes. Such lasers are actually very interesting, partly because they can be developed at different wavelengths, e.g. for blue light generation with intracavity frequency doubling at very high output power levels.

Gain Media

Definition: media for laser amplification

Within the context of laser physics, a laser gain medium is a medium which can amplify the power of light (typically in the form of a light beam). Such a gain medium is required in a laser to compensate for the resonatoractive laser medium. It can also be used for application in an optical amplifier. The term gain refers to the amount of amplification. losses, and is also called an

As the gain medium adds energy to the amplified light, it must itself receive some energy through a process called pumping, which may typically involve electrical currents (electrical pumping) or some light inputs (→ optical pumping), typically at a wavelength which is shorter than the signal wavelength.

Types of Laser Gain Media

There are a variety of very different gain media; the most common of them are:

• Certain direct-bandgap semiconductors such as GaAs, AlGaAs, or InGaAs are typically pumped with electrical currents, often in the form of quantum wells (→ semiconductor lasers).
• Certain laser crystals and glasses such as Nd:YAG (neodymium-doped yttrium aluminum garnet → YAG lasers), Yb:YAG (ytterbium-doped YAG), Yb:glass, Er:YAG (erbium-doped YAG), or Ti:sapphire are used in the form of solid pieces (→ bulk lasers) or optical glass fibers (→ fiber lasers, fiber amplifiers). These crystals or glasses are doped with some laser-active ions (in most cases trivalent rare earth ions, sometimes transition metal ions) and optically pumped. Lasers based on such media are sometimes called doped insulator lasers.
• There are ceramic gain media, which are also normally doped with rare earth ions.
• Laser dyes are used in dye lasers, typically in the form of liquid solutions.
• Gas lasers are based on certain gases or gas mixtures, typically pumped with electrical discharges (e.g. in CO₂ lasers and excimer lasers).
• More exotic gain media are chemical gain media (converting chemical energy to optical energy), nuclear pumped media, and undulators in free electron lasers (transferring energy from a fast electron beam to a light beam).

Compared with most crystalline materials, ion-doped glasses usually exhibit much broader amplification bandwidths, allowing for large wavelength tuning ranges and the generation of ultrashort pulses. Drawbacks are inferior thermal properties (limiting the achievable output powers) and lower laser cross sections, leading to a higher threshold pump power and (for passively mode-locked lasers) to a stronger tendency for Q-switching instabilities. See the article on laser crystals versus glasses for more details.

The doping concentration of crystals, ceramics and glasses often has to be carefully optimized. A high doping density may be desirable for good pump absorption in a short length, but may lead to energy losses related to quenching processes, e.g. caused by upconversion via clustering of laser-active ions and energy transport to defects.

Important Physical Effects

In most cases, the physical origin of the amplification process is stimulated emission, where photons of the incoming beam trigger the emission of additional photons in a process where e.g. initially excited laser ions enter a state with lower energy. Here, there is a distinction between four-level and three-level gain media.

A less frequently used amplification process is stimulated Raman scattering, involving the conversion of some higher-energy pump photons into lower-energy laser photons and phonons (related to vibrations e.g. of the crystal lattice).

For high levels of input light powers, the gain of a gain medium saturates, i.e., is reduced. This naturally follows from the fact that for a finite pump power an amplifier cannot add arbitrary amounts of power to an input beam. In laser amplifiers, saturation is related to a decrease in population in the upper laser level, caused by stimulated emission.

Thermal effects can occur in gain media, because part of the pump power is converted into heat. The resulting temperature gradients and also subsequent mechanical stress can cause lensing effects, distorting the amplified beam. Such effects can spoil the beam quality of a laser, reduce its efficiency, and sometimes even destroy the gain medium (thermal fracture).

Relevant Physical Properties of Laser Gain Media

A great variety of physical properties of a gain medium can be relevant for use in a laser. The desirable properties include:

• a laser transition in the desired wavelength region, preferably with the maximum gain occurring in this region
• a high transparency of the host medium in this wavelength region
• a pump wavelength for which a good pump source is available (in case of an optically pumped laser); efficient pump absorption
• a suitable upper-state lifetime: long enough for Q-switching applications, short enough if fast modulation of the power is required
• a high quantum efficiency, obtained via a low prevalence for quenching effects, excited-state absorptionmulti-phonon transitionsenergy transfers and the like, but also possibly by strong enough beneficial effects such as certain or
• ideally, four-level behavior, because quasi-three-level behavior introduces various additional constraints
• robustness and a long lifetime, chemical stability
• for solid-state gain media: a host medium which is available with good optical quality in the required size, can be cut and polished with good quality (appropriate hardness), allows for high doping with laser-active ions without clustering, is chemically stable (e.g., not hygroscopic), and has a good thermal conductivity and low thermo-optic coefficients (for weak thermal lensing in high-power operation) and high resistance to mechanical stress; optical isotropy can be desirable, but in other cases birefringence (reducing thermal depolarization) and possibly polarization-dependent gain is preferable (see also: polarization of laser emission)
• for high gain, low threshold pump power: a high product of emission cross section and upper-state lifetimeσ– product) (
• for low beam quality requirements on the pump source: high pump absorption may be helpful
• for wavelength tuning: a large gain bandwidth
• for ultrashort pulse generation: a broad and smoothly shaped gain spectrum; suitable chromatic dispersionnonlinearity are also sometimes important and
• for passive mode locking without Q-switching instabilities: high enough laser cross sections
• for high energy pulse amplification (e.g. in regenerative amplifiers): a high optical damage threshold and not too high saturation fluence of the gain

Note that in many situations there are partially conflicting requirements. For example, a very low quantum defectgain bandwidth typically means that laser cross sections are smaller than ideal, and that the quantum defect cannot be very small. Disorder in solid-state gain media increases the gain bandwidth, but also reduces the thermal conductivity. A short pump absorption length can be advantageous, but also tends to exacerbate thermal effects. is not compatible with four-level behavior. A large

It is apparent that different situations lead to very different requirements on gain media. For this reason, a very broad range of gain media will continue to remain important for applications, and making the right choice is essential for constructing lasers with optimum performance.

Single-atom Lasers

Definition: lasers with only a single atom as the gain medium

Usually, the gain medium of a laser contains a huge number of laser-active atoms or ions. However, for various reasons it is of interest to study the behavior of a single-atom laser, where a single atom constitutes the whole gain medium. In 2003, Kimble's research group at the California Institute of Technology demonstrated the first realization of a single-atom laser (or one-atom laser) [3]. Cesium atoms were laser cooled and trapped in a magneto-optical trap (MOT) and then released in order to fall downwards. A single cesium atom was then loaded into a far-off-resonance optical trap (FORT), realized between two supermirrors which formed a high-finesse resonator serving as the laser resonator. The resonator length was actively stabilized using an auxiliary laser. The “inversion” of the cesium system (as far as this term makes sense in this regime) was also achieved with optical pumping. Laser emission occurred in the form of two Gaussian beams exiting the laser resonator at the end mirrors.

Such a one-atom laser is not just a miniaturized version of an ordinary laser. Due to the high cavity finesse and the small mode volume, this device operates in the unusual regime of strong coupling between the photons of the light field and the atomic transition. Specifically, the Rabi frequency is well above both the spontaneous emissionRabi cycles before the excitation decays. In this regime, the theoretical predictions from a full quantum description of the dynamics differ strongly from those of a semiclassical model (with classical treatment of the light field) as usually used to describe laser operation. rate and the photon decay rate of the cavity, so that the coupled system can undergo several

The investigation of such devices is of fundamental interest as it makes it possible to test certain predictions of quantum optics. Indeed, the experiments confirmed the prediction that a single-atom laser should have no laser threshold (→ thresholdless lasers), i.e. the laser emission occurs even for the smallest pump powers. Further, the laser output is not in a coherent state as for most other lasers, but consists of nonclassical light. In particular, significant photon antibunching and sub-Poissonian photon statistics could be observed with coincidence measurements, particularly at low pump rates. Much can be learned by comparing various experimental observations with predictions from laser models involving the quantized light–matter interaction. The opportunity to do detailed studies of all these effects makes such efforts worthwhile, even though it is hardly conceivable that a single-atom laser will find any practical application.

Note that there have been single-atom micromasers, where single atoms interacted with a microwave cavity. A significant difference, however, is that in this case one is usually dealing with an atomic beam where different atoms subsequently interact with the light field, even though at each time there is at most one atom in the cavity. In contrast, the single-atom laser really works with just one atom over longer times, as compared with, e.g., the inverse Rabi frequency.

Single-atom lasers should not be confused with atom lasers, which emit coherent matter waves rather than light.