Fabrication method and equipment for diffraction gratings

#The present invention relates to a method and equipment for fabricating diffraction gratings, and more particularly to a method and equipment for fabricating diffraction gratings by two-beam interference, that is adapted to produce a DFB (distributed feedback) laser which consists of a homogeneous diffraction grating for the light source of optical fiber communications.

#The conventional DFB laser shown in FIG. 1 operates at two different wavelengths at the same time, because the threshold currents for each wavelength .lambda. (or propagation constant .beta.) will degenerate with each other, as has been disclosed in H. Kogelnik et al., J. Appl. Phys., Vol. 43, pp. 2327-2335, 1972. To make a single longitudinal mode, therefore, it is an essential requirement to adjust the relative positions between the cleavage plane and the diffraction grating as has been experimentally demonstrated in T. Matsuoka et al., Jpn. Appl. Phys., Vol. 23, pp. L138-L140, 1984. To realize the single longitudinal mode, the positional precision of about.+-.100 angstroms is necessary, and it is difficult to adjust the relative positions relying upon the position of ordinarily employed cleavage. According to Haus et al.,IEEE J. Quantum Electronics, Vol. QE-12, pp. 532-539, 1976, on the other hand, it has been reported that the operation at the single longitudinal mode can be stably obtained if the diffraction grating is shifted in phase by .lambda./4 at nearly the center of the element. The diffraction grating having such a phase shift can be fabricated by a method of direct drawing based upon the electron beam disclosed in K. Sekartedjo et al., Electron Letter, 1983, or by a method which employs both a positive resist and a negative resist. With the method of direct drawing based upon the electron beam, however, the throughput is substantially small. With the method which employs a positive and negative resists, on a other hand, thickness of the resists becomes uneven, and the obtained diffraction gratings exhibit asymmetrical diffraction efficiency depending upon the right and left halves of the element, making it difficult to reproduce the single-mode operation.

#An interference method can also be used as shown in FIG. 3. In this case, the laser beam emitted from a laser beam source 11 such as He-Cd laser or argon laser is split through a beam splitter 12, the beams after being split, are expanded for their diameter through beam expanders 131, 132, reflected by mirrors 141, 142 to change their directions of propagation, and are projected onto the surface of a specimen 15 to obtain fringed interference pattern. According to this conventional method without having a mechanism for adjusting the phase for the two light beams however, it is not possible to deliberately obtain diffraction gratings having dissimilar phases while maintaining the same period.

#FIG. 1 is a section view showing a conventional DFB laser;

#FIG. 2 is a section view showing a DFB laser of the phase shift type;

#FIG. 3 is a schematic diagram of a conventional interference exposure apparatus for producing the phase shift-type DFB laser;

#With reference to FIG. 4, if a mechanism for causing a phase change is provided at any one-of the positions 21 22 and 23, or on a mirror 142, there can be obtained on the same substrate diffraction gratings having an equal grating distance but different phases. An example of the mechanism disposed at the position 21, 22 or 23 to cause a phase change is shown in detail in FIG. 5. The material 24 which causes a phase change is shown to include plate 24, and material 25 which covers only portions of plate 24'. That is, the phase can be shifted by disposing a material which has an index of refraction different from that of the surrounding air and which produces a suitable difference in the optical path at least in a portion of the surface perpendicular to the optical axis of at least one light beam.

#Described below is the case where a phase difference of .lambda./2 is produced in a portion of the light beam by installing at the position 23 of FIG. 4 an optical flat (1 cm thick) made of quartz which has an SiO.sub.2 coating of 1100 .ANG. when a He-Cd laser beam (.lambda.=3250 .ANG.) is used or of 1700 .ANG. when an Ar laser beam (.lambda.=5145 .ANG.) is used, placed on a portion on one surface thereof, both surfaces of the flat being coated with an anti-reflection coat.

#An InP substrate coated with an AZ-type resist is used as the specimen 15, and developing is effected using an AZ developer, to obtain a grating having different phases as a resist pattern. With the resist as a mask, the grating is transferred onto the InP substrate by using an etching solution composed of HBr:HNO.sub.3 :H.sub.2 O (=1:1:30). On an n-type InP substrate 1 on which is transferred the grating 2 with phase difference (FIG. 2) are formed, by the liquid-phase epitaxial method, an InGaAsP optical guide layer 3 (doped with tellurium, having a carrier concentration of 1.times.10.sup.18 cm.sup.-3, a thickness of about 0.1 .mu.m, and a calculated band gap wavelength of 1.3 .mu.m), an InGaAsP active layer 4 (undoped, having a thickness of about 0.1 .mu.m, and a calculated band gap wavelength of 1.5 .mu.m), an InGaAsP buffer layer 5 (doped with zinc, having a carrier concentration of 7.times.10.sup.17 cm.sup.-3, a thickness of about 0.1 .mu.m, and a calculated band gap wavelength of 1.3 .mu.m), a p-type InP layer 6 (doped with zinc, having a carrier concentration of 7.times.10.sup.17 cm.sup.-13, and a thickness of 3 .mu.m), and a p-type InGaAsP cap layer 7 (doped with zinc, having a carrier concentration of 5.times.10.sup.18 cm.sup.-3, a thickness of 0.4 .mu.m, and a calculated band gap wavelength of 1.5 .mu.m), and a p-type electrode 81 (Au/Cr) and an n-type electrode 82 (Au/Sn) which are deposited by vaporization, followed by cleavage. Then, an anti-reflection coat 9 (SiN) is formed on the cleavage planes to prepare a semiconductor laser element shown in FIG. 2. This element operates at Bragg wavelength on a single longitudinal mode, and exhibits the effect of shifting the phase.