JPH0258285A - Manufacture of diffraction grating - Google Patents

Abstract

PURPOSE:To obtain a manufacturing method of a diffraction grating for DFB laser, lambda/4 shift DFB laser, etc., which are stable, excellent in mass-productivity, and does not require much manhour by making the grating by coherent transfer method using a grating pattern composed of photo-setting resin made by using a grating pattern formed by charged beam drawing method, as an original plate. CONSTITUTION:A grating is made by the following manner; by charged beam drawing method, a grating pattern is formed on a resist layer 20 on a substrate 19; this grating pattern is used as an original plate, and a duplicate of the above grating pattern is made by using photo-setting resin 21; the grating is made by coherent transfer method using the grating pattern composed of the photo-setting resin 21. For example, by charged beam drawing method, a diffraction grating having a desired pitch is formed on the photoresist layer 20 on the substrate 19; by using the pattern on the substrate 19 as an original plate, a replica is made from the photo-setting resin; by using the grating transferred to the photo-setting resin 21 as a basic diffraction grating, coherent transfer on a semiconductor laser substrate, on which photoresist is applied, is performed.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a diffraction grating, particularly a distributed feedback (DFB).
distributed feed back, hereinafter DF
The present invention relates to a method for manufacturing a diffraction grating used in a (abbreviated as B) type semiconductor laser and a λ/4 shift DFB laser.

[Conventional technology]

DFB as a light source for high-speed, large-capacity optical cable communication
Development of a type semiconductor laser and a λ/4 shift DFB laser is progressing. Since these lasers oscillate in a single axis mode even during high-speed modulation, they can be combined with single-mode fibers to achieve the high-speed,
Large capacity can be achieved.

In a typical semiconductor laser, a Fabry-Perot resonator is formed between both ends of the active layer as a resonator, and between both folds. Light within the active layer is reflected by the inter-fold surfaces, travels back and forth within the active layer, and reaches laser oscillation.

On the other hand, in the case of a DFB laser, a diffraction grating is placed near the active layer, and a resonator is constructed using Bragg reflection of the diffraction grating. The light within the active layer is gradually reflected by the Bragg reflection of the diffraction grating, travels back and forth within the active layer, and reaches laser oscillation.

There is the following relationship between the pitch p of a diffraction grating used in a DFB laser and the oscillation wavelength λ.

λ　　　　λ.

Here, m is the order of the diffraction grating, λ is the oscillation wavelength, no is the equivalent refractive index within the element, and λ is the wavelength of light propagating within the element. The order m of the diffraction grating is usually l or 2. The lower the order, the smaller the grating pinch, making it more difficult to manufacture the diffraction grating, but it is desirable because the coupling coefficient between the light wave and the diffraction grating becomes larger. To give an example of lattice pinch, DF in the 1 and 3 μm band using InP
In the case of a B laser, it is approximately 0.2 μm for a first-order diffraction grating. Therefore, each of the concave and convex portions of the grating has a width of 0.1 μm.

A two-beam interferometry method using laser beams is a method for producing such submicron-pinch gratings with high precision. FIG. 2 is a diagram for explaining this method. The light emitted from the laser oscillator 1 is reflected by a mirror 7, separated into two by a beam splitter 2, and then sent to a beam expander 3,
4 and reflected by mirrors 5 and 6, the two light beams overlap and interfere on the substrate 8, exposing the photoresist 9 on the substrate. Here, by appropriately setting the angle of the light beam incident on the substrate 8, it is possible to create a photoresist grating with an arbitrary pitch. With this method, a grating having a pitch up to about 1/2 of the wavelength of the laser beam used can be created with high precision. A diffraction grating for a DFB laser is obtained by etching the semiconductor substrate using the created photoresist grating as a mask.

The λ/4 shift DFB laser has been developed with the aim of further improving the yield of single-axis mode oscillation than normal DFB lasers, and when using a first-order diffraction grating, the λ/4 shift DFB laser The phase of the diffraction grating is inverted.

(λ/4 shift) Various methods have been proposed and experimented with to create diffraction gratings for DFB lasers, but basically they are based on the two-beam interferometry method explained earlier. A different phase shift section is used, or a negative resist and a positive resist are used. FIG. 3 is a diagram for explaining a typical method of creating a diffraction grating for a λ/4 shift DFB laser using a negative resist and a positive resist, and shows a cross section of the substrate. Note that in this figure, the optical system portion for two-beam interference exposure is omitted to simplify the explanation. In this method, the portion of the negative resist 11 on the substrate III that is irradiated with the laser beam becomes insoluble in the developer, while the positive resist 12 becomes soluble, so when the laser beam of the same phase is irradiated, the resist that remains after development It uses the fact that the concavities and convexities are each reversed.

[Problem to be solved by the invention]

Since the above-described method for producing a diffraction grating for a DFB laser basically uses interference of two light beams, it is necessary to form an interferometer, which has the drawback of increasing the size of the apparatus.

Furthermore, in order to form a submicron grating, it was necessary to eliminate vibrations within the interferometer as much as possible. Furthermore, in order to separate the laser emitted beam into two beams, if there is air fluctuation in one beam, a grating will not be formed, so it is necessary to suppress the movement of air within the interferometer.

Therefore, in mass production of DFB lasers, many man-hours are required to maintain and control the interferometer. Furthermore, in addition to the above-mentioned problems, fabrication of a diffraction grating for a λ/4 shift DFB laser requires a complicated resist process, requiring a huge number of man-hours and strict process control.

The purpose of the present invention is to solve the above-mentioned problems, and to provide a DFB laser which is stable, suitable for mass production, and does not require much man-hours.
Another object of the present invention is to provide a method for manufacturing a diffraction grating for a λ/4 shift DFB laser.

[Means to solve the problem]

The method for manufacturing a diffraction grating of the present invention includes using a grating pattern formed on a resist layer on a substrate by a charged beam drawing method as a master, creating a copy of the grating pattern using a photocurable resin, and making a copy of the grating pattern using a photocurable resin. It is characterized by creating a lattice using a coherent transfer method using a lattice pattern.

[Effect]

The operation of the present invention will be described below with reference to the drawings. In the present invention, a coherent transfer method is used to create a diffraction grating without dividing the beam emitted from a laser oscillator into two by a beam splitter. FIG. 4 is a diagram for explaining the coherent transfer method. When the laser beam 13 is irradiated onto the diffraction grating 14, along with the O-order diffracted light, ±1
A second diffracted light is generated. Here, when light is incident on the diffraction grating 14 at a Bragg angle θ6, the +1st-order diffracted light 15 and the 11st-order diffracted light 16 are emitted at the same angle (Bragg angle θ,) with respect to the normal to the diffraction grating 14. That is, in the very vicinity of the exit side plane of the diffraction grating, interference is observed between two light beams that intersect at equal angles to the normal line of the diffraction grating. Therefore, as shown in FIG. 5, if a substrate 18 coated with a photoresist 17 is placed within this area, this interference pattern will be recorded on the photoresist. Hereinafter, the grating used to generate these two diffracted lights will be referred to as a basic diffraction grating.

In the present invention, a grating pattern is drawn on a photoresist using a charged beam such as an electron beam or an ion beam, and a replica of the grating made of this resist is created and used as a basic diffraction grating. By using a pattern drawn with a charged beam as a master, parameters of the basic diffraction grating such as grating depth and grating pitch can be controlled arbitrarily, making it easy to create a basic diffraction grating with desired characteristics.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 6 is for explaining the manufacturing procedure of the basic diffraction grating in the present invention. First, as shown in FIG. 6(a), a diffraction grating having a desired pinch is formed on the photoresist layer 20 on the substrate I9 by the charged beam m-painting method. The cross-sectional shape of the diffraction grating created here is almost rectangular, but
If you want to obtain a triangular cross-sectional shape due to demands for diffraction efficiency, etc., use a semiconductor crystal as the substrate, perform wet etching using the lattice pattern created here as a mask, and take advantage of the anisotropy of the etching rate. , is possible.

Next, as shown in FIG. 6(b), a replica is created using a photocurable resin 21 using the pattern on this substrate as a master. The grating transferred to this photocurable resin is used as a basic diffraction grating.

In the basic diffraction grating manufacturing procedure described above, it is of course possible to create an electroforming mold of the grating pattern on the substrate and use this mold as a master to transfer the pattern to the photocurable resin. In the above-mentioned method, once a replica is created using photocurable resin, the master is destroyed, but in the method using electroforming molds, the master is preserved, making it possible to mass produce replicas, or in other words, to provide a stable supply of basic diffraction gratings. This has the advantage that the diffraction grating can be manufactured stably.

FIG. 1 shows an optical system for coherent exposure in the present invention.

The light emitted from the laser oscillator 22 is passed through a shutter 23 for controlling exposure time and an ND filter (ne
utral density filter) 24, and an objective lens 25. Spatial filter 26. The beam is expanded by a beam expander 29 consisting of a collimating lens 27 to become parallel light, which is incident on the basic diffraction grating 30 created by the above-described procedure. The basic diffraction grating is arranged so that the incident parallel light has a Bragg angle.

A photoresist 3 is placed very close to the rear of the basic diffraction grating 30.
The semiconductor laser substrate 32 coated with 1 is the basic diffraction grating 31.
is placed parallel to. Using this optical system,
By appropriately adjusting the exposure time and laser light intensity,
A desired diffraction grating can be created in a photoresist layer on a semiconductor substrate.

〔Effect of the invention〕

According to the present invention, diffraction gratings for DFB lasers and λ/4 shift DFB lasers can be stably manufactured using a very simple manufacturing procedure, so the man-hours required for manufacturing diffraction gratings can be significantly reduced. This contributes to lowering the cost of these lasers.

Further, the method for manufacturing a diffraction grating according to the present invention can be applied not only to the above-mentioned laser, but also to the manufacture of a diffraction grating that was conventionally manufactured by two-beam interferometry.

[Brief explanation of the drawing]

1 and 6 are diagrams for explaining the present invention in detail, Figures 2 and 3 are diagrams for explaining the conventional technology, and Figures 4 and 5 are diagrams for explaining the present invention in detail. It is a figure for explaining. ■, 22...Laser oscillator 2...Beam splitter 3.4.29...Beam expander 5.6.7...
・Mirror 8, 10.18.19...Substrate 9, 17.31...Photoresist 11...Negative resist 12...Positive resist 13...・Laser beam 14...Diffraction grating 15...+1st order diffraction grating 16...-1st order diffraction grating 20...Photoresist layer 21 ......Photocurable resin 23...Shutter 24...ND filter 25...
...Objective lens 26, 27, 30, 32, Spatial holder, Collimating lens, Basic diffraction grating, Semiconductor laser substrate

Claims (1)

[Claims] (1) Using a lattice pattern formed on a resist layer on a substrate by a charged beam drawing method as a master, a copy of the lattice pattern is created using a photocurable resin, and the lattice pattern made of this photocurable resin is used for coherent transfer. 1. A method for manufacturing a diffraction grating, which comprises creating a grating by a method.