Semiconductor laser

Details Semiconductor laser Semiconductor laser

2002-05-10

2004-04-20

Ip, Paul (Department: 2828)

Coherent light generators

Particular active media

Semiconductor

C372S046012, C372S043010, C372S039000

Reexamination Certificate

active

06724795

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to semiconductor lasers, and more particularly to high power semiconductor lasers suitable for optical telecommunication applications.
BACKGROUND OF THE INVENTION
Semiconductor lasers are typically formed from pn-junctions that have been enhanced to facilitate the efficient recombination of electron-hole pairs leading to the emission of radiation (light energy). A well known improvement to semiconductor lasers was the addition of a new layer of material between the P-type and N-type semiconductor layers, the new layer of material having a lower band gap energy than P-type and N-type layers. The layer formed by the material having the lower band gap energy is commonly referred to as the active region (or active layer) in a semiconductor laser.
Typically, a heterojunction refers to an interface between two different materials. Therefore, the insertion of an extra layer (active region) between the P-type and N-type layers results in what is known as a double heterostructure, as there will be a heterojunction at the interface of both the P-type and N-type materials. The doping in the active region is set at various levels depending upon the effect it is intended to have.
Thus, it is now common practice for semiconductor heterostructure lasers to be made up of three or more semiconductor layers. The simplest lasers include a P-type confinement region (P-type layer), an N-type confinement region (N-type layer) and an active region. The active region is typically made up of a number of layers and is located in the depletion region of the pn-junction between the P-type and N-type confinement regions. The optical mode is primarily confined in the active region because of the difference in the index of refraction between the active region, and the P-type and N-type confinement regions. The active region provides gain to the optical mode when the heterostructure is forward biased.
It is within the active region where light is generated once the semiconductor laser is forward biased and current is injected into the heterostructure. The active region is often composed of many layers in order to tailor the performance of the laser to meet the desired requirements (e.g. modulation bandwidth, power, sensitivity to temperature, etc.) of the laser's intended application.
The maximum optical output power of a semiconductor laser is usually limited by heating. The temperature of the active region increases with drive current, which degrades the laser performance. To achieve high optical power, one usually needs to increase the cavity length and the ridge width, which decreases the dissipated power density and keeps the laser from over heating. The power density is decreased because the electrical and thermal impedances decrease as the area where the current is injected increases.
When the cavity length is increased (typical cavity length is 2 mm for a hitch power laser), the efficiency (mW of optical power/mA of drive current) decreases because of internal optical loss in the cavity (that is not particular to the ridge structure, but is common in all structures). The optical loss is mainly clue to the absorption of the light energy in the P-type material (region). Decreasing the overlap of the optical mode within the P-type region would then be a useful way to decrease the loss of light energy within the laser, which would enable the use of longer cavities to be used to create lasers with higher output power.
There are different structures that can be used to decrease the optical losses (i.e. losses of light energy). However, those structures usually decrease the optical mode size in the laser cavity. The drawback is that the far field of the optical mode (i.e. optical far field) gets wider and the optical power is more difficult to couple into an optical fiber. The optical fat field and the optical mode in the laser cavity (the near field) are mathematically related by Fourier transform. This is a consequence of optical diffraction. Usually the optical far field is symmetric even though the near field is not. The loss in the coupling efficiency into the fiber happens only because the optical mode in the fiber and the laser far field do not have the same shape. An optical fiber can only accept a circular spot with a maximal divergence. The laser tar field is usually elliptical and can have a large divergence.
For telecommunication applications it is the amount of optical power coupled into the fiber and not the raw optical power out of the laser that is significant. Thus, there is a need for a structure that simultaneously: 1) has low optical losses, so that a long cavity can be used to achieve high output power; 2) maintains a low divergence so that there is more power of the elliptical far field coupled into the optical fiber.
The active region is commonly made up of a number of layers, some of which are designed to be quantum wells (or bulk wells). A quantum well is designed to be a very thin layer, thus allowing a better localization of electrons in the conduction band and holes in the valence band that will enhance electron-hole pair recombination. When an electron-hole pair recombine the excess energy the electron had possessed is emitted as light (radiation) adding to the operation of the laser. Furthermore, reducing the band gap energy of the active region relative to the band gap energies of the two confinement layers improves the confinement of the electrons and holes to the active region; thus, the optical mode profile is guided to remain within a narrow spot. However, for lasers suitable for optical telecommunications, an optical mode profile that is too narrowly confined is difficult to couple into a fiber as it will have a wide far field.
To achieve the best performance in a high-power laser, both the internal and external efficiency of the laser must be maximized. The internal efficiency of a laser is the efficiency at which electrical energy is converted into light energy (i.e. into the optical mode). The external efficiency is the efficiency at which the optical mode leaves the laser. However, there is a trade-off between the two measures of efficiency and thus far high power lasers have been limited by this trade off. Specifically, when considering semiconductor lasers, the external efficiency is largely the result of optical mode energy losses in P-type confinement layer, which tends to absorb much more optical energy than the active or N-type layers. On the other hand, internal efficiency (of semiconductor lasers) is usually dominated by current leakage which increases with temperature, and the temperature in turn increases with drive current. In other words, the electrical energy supplied to the laser is not maximally converted into optical energy within the laser as some current is dissipated through the semiconductor layers.
There is also another significant source of optical energy loss that must be taken into account when considering lasers for optical telecommunication applications. Semiconductor lasers used for optical telecommunication applications must hare their outputs coupled to a fiber and as such it is common that lasers are commercially packaged with a short piece of fiber, known as a pigtail, already aligned to the output of the laser. Thus, for telecommunication applications the external efficiency of a laser should be measured to include the effects of industrial packaging. In this case that would mean that the external efficiency of a laser should be measured at the end of the pigtail so that coupling losses can be taken in account. In other words, the potential for coupling loss from the laser into the pigtail must be considered in the design of a laser to be used for optical telecommunication applications as coupling loss can be a significant contributor to the degradation of the external efficiency. Precise alignment of the laser output to the pigtail is not enough to solve this problem. Current high-power lasers have outputs that have a wide far field, due to attempts to confine the optical mode in the

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