半导体激光器

维基百科，自由的百科全书

-{T|zh-cn:半导体激光器;zh-tw:雷射二極體}-

半导体激光器 是指以类似于發光二極體的半導體材料為激發介質的雷射。最普通实用的半导体激光器由一个P-N結组成，并由注入的电流驱动。这些设备有时被称为注入式激光器以区别于更早在实验室里生产的光抽运激光器。

目录 1 運作原理 2 雷射二極體的類型 2.1 雙異質結構雷射(Double heterostructure lasers) 2.2 量子井雷射 2.3 Separate confinement heterostructure lasers 2.4 分散回饋雷射(Distributed feedback lasers) 2.5 垂直共振面発光雷射(VCSELs) 2.6 VECSELs 3 雷射二極體的用途 4 損壞形式 5 See also 6 參考資料 7 外部連結

[编辑] 運作原理

雷射二極體，如同其他的半導體元件，是藉由在晶體表面生成一個非常薄的參雜層所製成。參雜後的晶體產生N型或P型區域，兩者重疊，就得到一個 PN接合，或二極體。

和其他的二極體一樣，當這個結構被施與順向偏壓，來自P型區域的電洞會被射入N型區域，此區的主要載子是電子；同樣地，來自N型區域的电子也被射入P型區域，此區電洞是主要載子。當電子和電洞出現在同一個區域，會互相結合並產生spontaneous emission--也就是說，電子進入電動的能態，並放射出與兩者能態差的相等能量的光子。被射入的電子和電洞表現為二極體的injection current，spontaneous emission的限制使得雷射二極體在激發門檻(lasing threshold)以下時與发光二极管有相似的性質。發生Spontaneous emission與否是啟動雷射震盪的先決條件，但是開始震盪後，由此放射出的能量是很微小的。

在適當的條件下，同區域內的電子和電洞在自然結合前可以共存一小段時間（以微秒計算）。若附近有能量與結合能量相等的光子，就可以透過stimulated emission效應引發電子電洞結合。這會產生出另一個與第一個光子相同波長、相同行進方向、相同偏振和相位的光子。這表示stimulated emission可以在injection區域產生（正確波長的）光波增益的效果，且通過接合區的電子、電洞增加，此增益也隨之增強。spontaneous 和 stimulated emission 在direct bandgap半導體中的效應比在indirect bandgap半導體中更為顯著，因此矽並不是製作雷射二極體常見的材料。

As in other lasers, the gain region is surrounded with an optical cavity to form a laser. In the simplest form of laser diode, an optical waveguide is made on that crystal surface, such that the light is confined to a relatively narrow line. The two ends of the crystal are cleaved to form perfectly smooth, parallel edges, forming a Fabry-Perot resonator. Photons emitted into a mode of the waveguide will travel along the waveguide and be reflected several times from each end face before they are emitted. As a light wave passes through the cavity, it is amplified by stimulated emission, but light is also lost due to absorption and by incomplete reflection from the end facets. Finally, if there is more amplification than loss, the diode begins to "lase".

Some important properties of laser diodes are determined by the geometry of the optical cavity. Generally, in the vertical direction, the light is contained in a very thin layer, and the structure supports only a single optical mode in the direction perpendicular to the layers. In the lateral direction, if the waveguide is wide compared to the wavelength of light, then the waveguide can support multiple lateral optical modes, and the laser is known as "multi-mode". These laterally multi-mode lasers are adequate in cases where one needs a very large amount of power, but not a small diffraction-limited beam; for example in printing, activating chemicals, or pumping other types of lasers.

In applications where a small focused beam is needed, the waveguide must be made narrow, on the order of the optical wavelength. This way, only a single lateral mode is supported and one ends up with a diffraction limited beam. Such single spatial mode devices are used for optical storage, laser pointers, and fiber optics. Note that these lasers may still support multiple longitudinal modes, and thus can lase at multiple wavelengths simultaneously.

The wavelength emitted is a function of the band-gap of the semiconductor and the modes of the optical cavity. In general, the maximum gain will occur for photons with energy slightly above the band-gap energy, and the modes nearest the gain peak will lase most strongly. If the diode is driven strongly enough, additional side modes may also lase. Some laser diodes, such as most visible lasers, operate at a single wavelength, but that wavelength is unstable and changes due to fluctuations in current or temperature.

Due to diffraction, the beam diverges (expands) rapidly after leaving the chip, typically at 30 degrees vertically by 10 degrees laterally. A lens must be used in order to form a collimated beam like that produced by a laser pointer. If a circular beam is required, cylindrical lenses and other optics are used. For single spatial mode lasers, using symmetrical lenses, the collimated beam ends up being elliptical in shape, due to the difference in the vertical and lateral divergences. This is easily observable with a red laser pointer.

The simple diode described above has been heavily modified in recent years to accommodate modern technology, resulting in a variety of types of laser diodes, as described below.

[编辑] 雷射二極體的類型

The simple laser diode structure, described above, is extremely inefficient. Such devices require so much power that they can only achieve pulsed operation without damage. Although historically important and easy to explain, such devices are not practical.

[编辑] 雙異質結構雷射(Double heterostructure lasers)

The first laser diode to achieve continuous wave operation was a double heterostructure demonstrated essentially simultaneously by Zhores Alferov of the Soviet Union, and Morton Panish and Izuo Hayashi working in the United States . In these devices, a layer of low bandgap material is sandwiched between two high bandgap layers. One commonly-used pair of materials is gallium arsenide (GaAs) with aluminium gallium arsenide (Al_xGa_(1-x)As). Each of the junctions between different bandgap materials is called a heterostructure, hence the name "double heterostructure laser" or DH laser. The kind of laser diode described in the first part of the article may be referred to as a homojunction laser, for contrast with these more popular devices.

The advantage of a DH laser is that the region where free electrons and holes exist simultaneously—the "active" region—is confined to the thin middle layer. This means that many more of the electron-hole pairs can contribute to amplification—not so many are left out in the poorly amplifying periphery. In addition, light is reflected from the heterojunction; hence, the light is confined to the region where the amplification takes place.

[编辑] 量子井雷射

If the middle layer is made thin enough, it acts as a quantum well. This means that the vertical variation of the electron's wavefunction, and thus a component of its energy, is quantised. The efficiency of a quantum well laser is greater than that of a bulk laser because the density of states function of electrons in the quantum well system has an abrupt edge that concentrates electrons in energy states that contribute to laser action.

Lasers containing more than one quantum well layer are known as multiple quantum well lasers. Multiple quantum wells improve the overlap of the gain region with the optical waveguide mode.

Further improvements in the laser efficiency have also been demonstrated by reducing the quantum well layer to a quantum wire or to a "sea" of quantum dots.

In a quantum cascade laser, the difference between quantum well energy levels is used for the laser transition instead of the bandgap. This enables laser action at relatively long wavelengths, which can be tuned simply by altering the thickness of the layer. As of 2005, quantum cascade lasers have not yet been widely commercialized.

[编辑] Separate confinement heterostructure lasers

The problem with the simple quantum well diode described above is that the thin layer is simply too small to effectively confine the light. To compensate, another two layers are added on, outside the first three. These layers have a lower refractive index than the centre layers, and hence confine the light effectively. Such a design is called a separate confinement heterostructure (SCH) laser diode.

Almost all commercial laser diodes since the 1990s have been SCH quantum well diodes.

[编辑] 分散回饋雷射(Distributed feedback lasers)

Distributed feedback lasers (DFB) are the most common transmitter type in DWDM-systems. To stabilize the lasing wavelength, a diffraction grating is etched close to the p-n junction of the diode. This grating acts like an optical filter, causing only a single wavelength to be fed back to the gain region and lase. Thus at least one facet of a DFB is anti-reflection coated. The DFB laser has a stable wavelength that is set during manufacturing by the pitch of the grating, and can only be tuned slightly with temperature. Such lasers are the workhorse of demanding optical communication

[编辑] 垂直共振面発光雷射(VCSELs)

Vertical cavity surface emitting lasers (VCSELs) have the optical cavity axis along the direction of current flow rather than perpendicular to the current flow as in conventional laser diodes. The active region length is very short compared with the lateral dimensions so that the radiation emerges from the ‘‘surface’’ of the cavity rather than from its edge as shown in Fig. 2. The reflectors at the ends of the cavity are dielectric mirrors made from alternating high and low refractive index quarter-wave thick multilayer.

Such dielectric mirrors provide a high degree of wavelength-selective reflectance at the required free surface wavelength λ if the thicknesses of alternating layers d₁ and d₂ with refractive indices n₁ and n₂ are such that n₁d₁ + n₂d₂ = (1/2)λ which then leads to the constructive interference of all partially reflected waves at the interfaces. Because of the high mirror reflectivities, VCSELs have lower output powers when compared to edge emitting lasers.

[编辑] VECSELs

Vertical external-cavity surface-emitting lasers, or VECSELs, are similar to VCSELs. In VCSELs, the mirrors are typically grown epitaxially as part of the diode structure, or grown seperately and bonded directly to the semiconductor containing the active region. VECSELs are distinguished by a construction in which one of the two mirrors is external to the diode structure. As a result, the cavity includes a free-space region. A typical distance from the diode to the external mirror would be 1 cm. Several workers demonstrated optically pumped VECSELs, and they continue to be developed for many applications including high power sources for use in industrial machining (cutting, punching, etc.) because of their unusually high power and efficiency when pumped by multi-mode diode laser bars.

Electrically pumped VECSELs have also been demonstrated. Applications for electrically pumped VECSELs include projection displays, served by frequency doubling of near-IR VECSEL emitters to produce blue and green light.

[编辑] 雷射二極體的用途

Laser diodes are numerically the most common type of laser, with 2004 sales of approximately 733 million diode lasers (Steele 2005), as compared to 131,000 of other types of lasers (Kincade and Anderson 2005).

Laser diodes find wide use in telecommunication as easily modulated and easily coupled light sources for fiber optics communication. They are used in various measuring instruments, eg. rangefinders. Another common use is in barcode readers. Visible lasers, typically red but recently also green, are common as laser pointers. Both low and high-power diodes are used extensively in the printing industry both as light sources for scanning (input) of images and for very high-speed and high-resolution printing plate (output) manufacturing. Infrared and red laser diodes are common in CD players, CD-ROMs and DVD technology. Blue-violet lasers will find their use in upcoming HD-DVD and Blu-Ray technology. High-power laser diodes are used in industrial applications such as heat treating, cladding, seam welding and for pumping other lasers (see list of laser types). The use of diode lasers for high-speed, low-cost, combustion spectroscopy is being explored.

In general, applications of laser diodes can be categorized in various ways. Most applications of diode lasers can be served by larger solid state lasers or optical parametric oscillators but it is the ability to mass-produce diode lasers at low cost that makes them essential for mass-market applications. Diode lasers have application to virtually every field of endeavor that attracts wide attention today. Since light has many different properties (power, wavelength & spectral quality, beam quality, polarization, etc.) it is interesting to classify applications by these basic properties.

Many applications of diode lasers primarily make use of the "directed energy" property of an optical beam. In this category one might include the laser printers, bar-code readers, image scanning, illuminators, designators, optical data recording, combustion ignition, laser surgery, industrial sorting, industrial machining, and directed energy weaponry. Some of these applications are emerging whereas many are familiar to the wider society.

Applications which may today or in the future make use of the "coherent" properties of diode-laser-generated light include interferometric distance measurement, holography, coherent communications, and coherent control of chemical reactions.

Applications which may make use of "narrow spectral" properties of diode lasers include telecommunications, infra-red countermeasures, spectroscopic sensing, generation of radio-frequency or terahertz waves, atomic clock state preparation, quantum key cryptography, frequency doubling and conversion, water purification (in the UV), and photodynamic therapy (where a particular wavelength of light would cause a substance such as porphyrin to become chemically active as an anti-cancer agent only where the tissue is illuminated by light).

Applications where the ability to "generate ultra-short pulses of light" by the technique known as "mode-locking" include clock distribution for high-performance integrated circuits, high-peak-power sources for laser-induced breakdown spectroscopy sensing, arbitrary waveform generation for radio-frequency waves, photonic sampling for analog-to-digital conversion, and optical code-division-multiple-access systems for secure communication.

[编辑] 損壞形式

Laser diodes have similar reliability and failure issues as light emitting diodes. In addition, they are subject to catastrophic optical damage (COD) when operated at higher power.

Many of the advances in reliability of diode lasers in the last 20 years remains proprietary to its developers. The reliability of a laser diode can make or break a product line. Moreover, "reverse engineering" is not always able to uncover the differences between more-reliable and less-reliable diode laser products.

At the edge of a diode laser, where light is emitted, a mirror is traditionally formed by cleaving the semiconductor wafer to form a specularly reflecting plane. This approach is facilitated by the weakness of the [110] crystallographic plane in III-V semiconductor crystals (such as GaAs, InP, GaSb, etc.) compared to other planes. A scratch made at the edge of the wafer and a slight bending force causes a nearly atomically perfect mirror-like cleavage plane to form and propagate in a straight line across the wafer.

But it so happens that the atomic states at the cleavage plane are altered (compared to their bulk properties within the crystal) by the termination of the perfectly periodic lattice at that plane. Surface states at the cleaved plane, have energy levels within the (otherwise forbidden) bandgap of the semiconductor.

Essentially as a result, when light propagates through the cleavage plane and transits to free space from within the semiconductor crystal, a fraction of that light energy is absorbed by the surface states whence it is converted to heat by phonon-electron interactions. This heats the cleaved mirror. In addition, the mirror may heat simply because the edge of the diode laser—which is electrically pumped—is in less-perfect contact with the mount that provides a path for heat removal. The heating of the mirror causes the bandgap of the semiconductor to shrink in the warmer areas. The bandgap shrinkage brings more electronic band-to-band transitions into alignment with the photon energy causing yet more absorption. This is thermal runaway, a form of positive feedback, and the result can be melting of the facet, known as catastrophic optical damage, or COD.

In the 1970's, this problem, which is particularly nettlesome for GaAs-based lasers emitting between 1 µm and 0.630 µm wavelengths (less so for InP based lasers used for long-haul telecommunications which emit between 1.3 µm and 2 µm), was identified. Michael Ettenberg, a researcher and later Vice President at RCA Laboratories' David Sarnoff Research Center (today known as Sarnoff Corporation) in Princeton, New Jersey, devised a solution. A thin layer of aluminum oxide was deposited on the facet. If the aluminum oxide thickness is chosen correctly little reflectivity at the wavelength of emission and is referred to as an "AR" coating (anti-reflect coating). This alleviated the heating and COD at the facet.

Since then, various other refinements have been employed. One approach is to create a so-called non-absorbing mirror (NAM) such that the final 10 µm or so before the light emits from the cleaved facet are rendered non-absorbing at the wavelength of interest.

In the very early 1990s, SDL, Inc. began supplying high power diode lasers with good reliability characteristics. CEO Donald Scifres and CTO David Welch presented new reliability performance data at, e.g., SPIE Photonics West conferences of the era. The methods used by SDL to defeat COD were considered to be highly proprietary and have still not been disclosed publicly as of June, 2006.

In the mid-1990s, IBM Research (Ruschlikon, Switzerland) announced that it had devised its so-called "E2 process" which conferred extraordinary resistance to COD in GaAs-based lasers. This process, too, has never been disclosed as of June, 2006.

Reliability of high-power diode laser pump bars (employed to pump solid state lasers) remains a difficult problem in a variety of applications, in spite of these proprietary advances. Indeed, the physics of diode laser failure is still in the process of being worked out and research on this subject remains active, if proprietary.

Extension of the lifetime of laser diodes is critical to their continued adaptation to a wide variety of applications.

[编辑] See also

• Laser diode rate equations
• Collimator
• Superluminescent diode

[编辑] 參考資料

• Kincade, Kathy and Stephen Anderson (2005) "Laser Marketplace 2005: Consumer applications boost laser sales 10%", Laser Focus World, vol. 41, no. 1. (online)
• Steele, Robert V. (2005) "Diode-laser market grows at a slower rate", Laser Focus World, vol. 41, no. 2. (online)

[编辑] 外部連結

• Sam's Laser FAQ by Samuel M. Goldwasser
• FAQ on Laser Diodes by Power Technology, Inc.
• Britney Spears Guide to Semiconductor Physics Edge-emitting lasers