Increasing the yield of precise wavelength lasers

Details Increasing the yield of precise wavelength lasers Increasing the yield of precise wavelength lasers

2002-08-24

2004-02-24

Fahmy, Wael (Department: 2813)

Semiconductor device manufacturing: process

Making device or circuit emissive of nonelectrical signal

Including integrally formed optical element

C438S047000

Reexamination Certificate

active

06696311

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to longitudinal mode laser diodes and, more particularly, to achieving higher yields of devises lasing at a desired wavelength.
BACKGROUND OF THE PRIOR ART
Existing semiconductor laser fabrication processes have difficulty in achieving devices that oscillate at a precise wavelength. One of the contributing factors is a variation of the material gain function that may arise during epitaxial crystal growth. Typical variation in lasing wavelength of lasers made from different growths may amount to +/− 5 nm. However many applications require lasers to have a wavelength accuracy of +/− 1 nm or better, e.g., for the optical pumping of Nd:YAG lasers at 808 nm. Other factors such as stress, temperature and thermal non-uniformities often cause additional variable shifts. The mismatch between the achievable wavelength variation and end-user specifications has led to difficulties in achieving desirable manufacturing yields.
The most common type of laser diode structure is the single quantum well which uses a ternary or quaternary material such as AlGaAs or GalnAsP in which a narrow bandgap, optically active, thin layer (
~
0.05 &mgr;m thick) is sandwiched between a pair of thicker, wider-bandgap waveguide and cladding layers. A semiconductor, edge-emitting (longitudinal mode) diode laser typically includes a resonator formed of a solid state laser gain medium extending longitudinally between input and output mirror surfaces usually formed by cleaving. The distance between the facets defines a F-P (F-P) cavity which is capable of sustaining several different longitudinal lasing modes. The gain medium is typically enclosed on laterally adjacent sides by reflective material having an index of refraction n
2
which is greater than the index of refraction n
1
of the gain medium material.
To improve wavelength stability, feedback can be provided by locating a first or second-order grating of suitable pitch either within the gain region of the solid state structure (for the DFB laser), or externally thereto (for the DBR laser). For example, U.S. Pat. No. 4,178,604 issued Dec. 11, 1979 shows a laser diode operating at 0.875 &mgr;m stabilized by a first order grating having a pitch &Lgr;=0.123 &mgr;m and an index of refraction n≅3.55 located between the active layer (n=3.6) and another layer (n=3.36). The grating is etched in a process using ultraviolet light to holographically etch a semiconductor layer located 0.3 &mgr;m away from the 0.1 &mgr;m thick active layer and having a refractive index lower than that of the active layer using interfering ultra-violet beams. The grating can be termed a “first order” grating since the pitch &Lgr;=0.123 &mgr;m is related to the desired laser wavelength &lgr;=0.875 &mgr;m by &Lgr;=&lgr;/2n. Unfortunately, ultraviolet light holography requires extensive measure to prevent unwanted carbon coating of optics due to photolyzation of organic vapors in the air which generally dictates that the process be performed in an extensively purged environment. Moreover, the surface of the mirror used to reflect the interfering waves must be extremely smooth to avoid unwanted scattering that would detract from the precise exposure of the photoresist, scattering being proportional to the inverse-fourth power of the wavelength of light employed. Finally, UV lasers are notoriously unreliable. It would be extremely desirable to obtain a manufacturing process that did not require the use of lasers that produce UV light.
Another example of a DFB laser appears in vol. 18 Electronics Letters for Jan. 7, 1982, at pp. 27, 28 which shows a GaInAsP/InP laser operating at a wavelength of 1.5 &mgr;m using a second order grating (&Lgr;=0.4522&mgr;) etched into an n-InP substrate. The grating was buried in a heterostructure comprised of a 0.17 &mgr;m thick waveguide layer of Sn-doped, n-GaInAsP adjacent to a non-doped, 0.19 &mgr;m thick GaInAsP active layer. To suppress unwanted F-P, modes the rear facet of the cavity was inclined.
U.S. Pat. No. 4,704,720 issued Nov. 3, 1987 asserted that the grating used in the aforementioned laser was located too far away from (i.e., too weakly coupled to) the optical field so that, at certain values of the injected current, oscillation at the unwanted Fabry-Perot modes occurred instead of at the desired single wavelength. Accordingly, the '720 patent laser, operating at an exemplary wavelength of 1.3 &mgr;m, located its second-order grating (having a pitch of &Lgr;=0.4&mgr;) in the strongest part of the optical field to obtain oscillation in a single longitudinal mode. Alternatively, a first order grating having a finer pitch (&Lgr;=0.2&mgr;) was suggested.
While a second-order grating can be produced using lower energy blue light beams in the photolithographic process, second-order gratings located in the high intensity optical field of the active layer or of the waveguide layer give rise to diffraction orders that sap energy from the single desired longitudinal mode. It would, however, facilitate ease of manufacture if blue light beams could be used to produce second-order gratings on wafers made from production runs of crystal growths having reasonable variation in their material gain function without incurring the penalty of energy loss through surface-diffraction losses. Wavelength stability is especially desirable at 0.808 &mgr;m which is the wavelength required to optically pump Nd:YAG lasers. One conventional arrangement has 19 laser gain stripes per bar so that it can be coupled to a bundle of 19 optical fibers, the output of which can be conveniently coupled into the Nd:YAG laser rod. It would be desirable to obtain a laser bar having sufficient wavelength stability among a comparable number of diodes on the bar to drive the fiber bundle.
In the copending application entitled “Increasing The Yield Of Precise Wavelength Lasers”, Ser. No. 09/848,529, filed May 3, 2001, there is disclosed an arrangement for overcoming the material gain function of the semiconductor material of the wafer that would tend to lase at an unwanted wavelength, &lgr;2. A second-order dielectric grating was embedded between epitaxial, gain-providing layers having different indices of refraction. The layer in which the grating was etched was located at a distance sufficiently remote from the high intensity optical field of the waveguide to provide just enough feedback to reduce the gain at the unwanted &lgr;2 wavelength and yet support oscillation at a desired wavelength &lgr;1 without incurring excessive surface diffraction loss. In the aforementioned patent application, feedback from the grating effected a gain discrimination factor, &agr; having an order of magnitude of 0.1 cm
−1
. The embedded grating provided stabilizing feedback and reduced the fraction of power lost to surface diffraction to less than 1 percent. When the wafer was processed into F-P laser devices, the coated facets provided most of the feedback, while the feedback from the grating altered the F-P longitudinal mode spectrum to create a preferred resonance condition.
While the aforementioned copending application provided a way to increase the yield of lasers having a precise wavelength, difficulties were encountered in growing the necessary epitaxial gain-providing layers over the etched grating layer. As is well known, growing epitaxial semiconductor layers that will exhibit the characteristics of a single crystal requires that extreme care be taken with respect to the surface on which the layers are to be grown. Unfortunately, etching processes tend to leave behind various oxides and impurities as well as surface defects that prevent the formation of single crystal growth structures on a disturbed surface. Moreover, the direction in which the grating lines are etched can also contribute to difficulties in obtaining overgrowth with precise characteristics.
To achieve the wavelength stability described in the aforementioned application, the grating line

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