Mode-hopping-free large-range continuous tunable external cavity semiconductor laser

Abstract

The invention provides a mode-hopping-free large-range continuous tunable external cavity semiconductor laser, which comprises a laser diode (1), a collimating lens (2), a polarization splitting prism (3), a narrow-band interference sheet (4), a prism-type super-strong-level reflective film (5) and a piezoelectric module (6), the laser diode (1), the collimating lens (2) and the polarization splitting prism (3) are sequentially arranged on a light path, and the narrow-band interference sheet (4), the prism-type super-strong-level reflective film (5) and the piezoelectric module (6) are bonded together. The rotation axis is located on the light path of the laser and coincides with the rear surface of the piezoelectric module (6). According to the mode-hopping-free large-range continuous tunable external cavity semiconductor laser, the sensitivity of the laser frequency to the rotation angle of the narrow-band interference sheet is utilized, and the angle and the cavity length of the interference sheet are changed at the same time, so that the transmission peak center frequency of the interference sheet and the specific cavity mode frequency are highly synchronized along with the change of the angle; according to the laser, mode hopping of the laser output frequency is avoided, and continuous tunability of the output frequency along with the rotation angle of the cavity mirror is guaranteed.

Description

Technical Field

[0001] The invention belongs to the technical field of semiconductor lasers, and in particular relates to a mode-hop-free, large-range continuously tunable external cavity semiconductor laser. Background Art

[0002] Semiconductor lasers are devices that inject current into semiconductor PN junctions, generate stimulated radiation through population inversion distribution, and combine optical feedback structures to generate and amplify light in PN junctions, ultimately forming laser output. However, semiconductor lasers generally have disadvantages such as unstable output lasers, large divergence angles, and large spectral linewidths. In order to optimize the performance of semiconductor lasers, external cavity semiconductor lasers extend the resonant cavity outside the semiconductor laser, effectively reducing the spectral linewidth of the laser, improving the beam quality, and reducing the divergence angle, making the output beam more stable and controllable, so as to meet the needs of application fields such as optical communications, lidar, and spectral analysis.

[0003] Existing interference plate external cavity semiconductor lasers, such as the interference plate laser based on the corner pyramid array external cavity reflector proposed in Chinese invention patent application CN 114899704 A, are implemented by outputting laser through a semiconductor laser, collimating it through a collimating lens and projecting it onto the corner pyramid array external cavity reflector to form a feedback laser, forming a resonant cavity between the semiconductor laser tube and the external cavity reflector, placing the interference plate between the collimating lens and the external cavity reflector, rotating their angles to obtain the target wavelength, bonding the piezoelectric ceramic to the external cavity reflector, and adjusting the laser cavity length by voltage to fine-tune the laser cavity length to change the laser resonant cavity length. However, the center frequency of the interference plate transmission peak does not change at the same time as the cavity mode frequency, which will cause mode hopping, limiting the continuous tunable range of the frequency.

[0004] Therefore, it is necessary to solve the defect that external cavity semiconductor lasers cannot be continuously tuned over a large range of frequencies, promote the application and development of laser technology in various fields, improve the performance of related technologies, and promote progress in scientific research, industrial production and other fields. Summary of the invention

[0005] The purpose of the present invention is to overcome the defects of the prior art and provide a mode-hop-free, wide-range continuously tunable external cavity semiconductor laser.

[0006] The idea of ​​the present invention is to utilize the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser, set up a set of devices capable of changing the cavity length, set the rotation axis on the rear surface of the cavity mirror with thickness, and change the cavity length by rotating the cavity mirror with thickness, thereby changing the fixed cavity mode frequency. At the same time, a narrow-band interference plate is used to assist in adjustment for mode selection, so that the center position of the transmission peak of the interference plate moves with the angle, and is highly synchronized with the change of the specific cavity mode frequency with the angle, so as to realize a laser whose laser output frequency does not jump mode and ensures the continuous tunability of the output frequency with the rotation angle of the cavity mirror, solve the problem of continuous frequency modulation, and finally realize a laser with wide range continuous frequency tunability.

[0007] Based on the above ideas, the present invention provides a mode-hopping-free, large-range, continuously tunable external cavity semiconductor laser, the laser comprising a laser diode 1, a collimating lens 2, a polarization beam splitter prism 3, a narrowband interference plate 4 and a prism-type super-strong level reflective film 5 arranged in sequence on an optical path, characterized in that the laser further comprises a piezoelectric module 6, the narrowband interference plate 4 is bonded to the front surface of the prism-type super-strong level reflective film 5, the piezoelectric module 6 is bonded to the rear surface of the prism-type super-strong level reflective film 5, the piezoelectric module 6 has a thickness D, and when the prism-type super-strong level reflective film 5 rotates, the rotation axis is located on the optical path of the laser and coincides with the rear surface of the piezoelectric module 6;

[0008] The coherent light beam emitted by the laser diode 1 is collimated into parallel light by the collimating lens 2. The parallel light is passed through the polarization beam splitter prism 3 to obtain horizontal polarized light or vertical polarized light with the same emission direction as the laser diode 1. The horizontal polarized light or vertical polarized light is incident on the narrow-band interference plate 4 to obtain narrow-band light. The narrow-band light reaches the front surface of the prism-type super-strong reflective film 5, is reflected, and returns to the laser diode 1. When the oscillation in the cavity reaches the threshold, the laser is output at the polarization beam splitter prism 3.

[0009] In the present invention, the thickness D of the piezoelectric module 6 satisfies the following relationship:

[0010]

[0011] Δλ=Δλ IF =λ0-λ′(φ)

[0012] And θ=φ

[0013] Wherein, Δλ is the change in the cavity mode wavelength when the prism-type super-strong reflective film 5 rotates by θ, Δλ IFis the wavelength change of the center position of the transmission peak of the interference plate when the narrow-band interference plate 4 rotates by φ, λ′(φ) is the wavelength of the center position of the transmission peak of the interference plate when the narrow-band interference plate 4 rotates by φ, λ0 is the transmission wavelength when the narrow-band interference plate 4 is perpendicular to the light beam, λ is the initial wavelength of the output of the laser diode 1, L1 is the distance from the output light end face of the laser diode 1 to the front surface of the prism-type super-strong grade reflective film 5 when the incident angle of the output light of the laser diode 1 to the prism-type super-strong grade reflective film 5 is 0°, D is the thickness of the piezoelectric module 6, θ is the incident angle of the prism-type super-strong grade reflective film 5, φ is the incident angle of the narrow-band interference plate 4, n eff is the refractive index of the narrow-band interference plate 4.

[0014] In the present invention, the initial wavelength of the laser diode 1 matches the transmission peak center wavelength of the narrow-band interference plate 4 .

[0015] The principle of the laser of the present invention is to achieve wide range continuous tunability by synchronously rotating the narrowband interference plate 4 and the prism-type super-strong reflective film 5 to adjust the interference plate transmission peak center frequency and the cavity mode frequency. The specific implementation principle is as follows:

[0016] When the laser is incident vertically on the front surface of the prismatic super-strong grade reflective film 5, that is, the incident angle is 0° (also called the initial angle of 0° in the present invention), assuming the thickness of the piezoelectric module 6 is D, the cavity length at this time is L1 (that is, the distance from the output light end face of the laser diode to the front surface of the prismatic super-strong grade reflective film 5).

[0017] (1) Given the initial cavity length L = L1, when the narrow-band interference plate 4, the prism-type super-strong reflective film 5 and the piezoelectric module 6 rotate together and the rotation angle is θ, the cavity length The change in cavity length caused by

[0018] (2) When the cavity length changes by half a wavelength, the frequency changes by c / 2L. Therefore, the relationship between the resonant cavity jitter ΔL and the cavity mode frequency change Δf is: Therefore, the relationship between the cavity mode frequency change Δf and the rotation angle θ is: Where f is the frequency, c is the speed of light, and λ is the initial wavelength of the output of laser diode 1.

[0019] (3) Known The relationship between the change in cavity mode wavelength and the thickness of the piezoelectric module 6 and its rotation angle θ is:

[0020] On the other hand, the standard formula for the relationship between the rotation angle φ of the narrow-band interference plate 4 (i.e., the incident angle of the narrow-band interference plate) and the wavelength change is: Where λ0 is the transmission wavelength when the narrowband interference plate 4 is perpendicular to the light beam, and the refractive index n of the narrowband interference plate 4 is eff The value is 2.

[0021] Therefore, in the present invention, by selecting a piezoelectric module 6 with an appropriate thickness D, when the interference plate and the rotating prism-type super-strong reflective film rotate synchronously, the change in the transmission wavelength caused by the rotation angle of the interference plate is Δλ IF (Δλ IF =λ0-λ′(φ)) is equal to or approximately the same as the change Δλ of the cavity mode wavelength caused by the rotation of the prism-type super-strong reflective film 5 (in the present invention, "approximately the same" is understood to mean that the error is extremely small and can be regarded as equal), thereby obtaining the effect of synchronous change of the central frequency of the interference plate transmission peak and the cavity mode frequency, solving the problems of small adjustable laser frequency range and discontinuous tunability caused by only changing the cavity length, and obtaining a wide range continuously tunable laser.

[0022] As another embodiment of the present invention, the thickness D satisfies the following relationship:

[0023] λ(θ)=λ′(φ) and θ=φ, where,

[0024]

[0025]

[0026] λ(θ) is the cavity mode wavelength when the prism-type super-strong grade reflective film (5) rotates by θ, λ′(φ) is the wavelength at the center position of the transmission peak of the interference plate when the narrow-band interference plate rotates by φ, θ is the incident angle of the prism-type super-strong grade reflective film (5), φ is the incident angle of the narrow-band interference plate (4), L1 is the distance from the output light end face of the laser diode (1) to the front surface of the prism-type super-strong grade reflective film (5) when the incident angle of the prism-type super-strong grade reflective film (5) is 0°, D is the thickness of the piezoelectric module (6), λ0 is the transmission wavelength when the narrow-band interference plate (4) is perpendicular to the light beam, and n eff is the refractive index of the narrowband interference plate (4), and n is a natural number.

[0027] That is, the prismatic super-strong level reflective film of appropriate thickness is selected through the relationship between the rotation angle of the prismatic super-strong level reflective film and the corresponding cavity mode wavelength, so that the change in the cavity mode wavelength caused by the rotating prismatic super-strong level reflective film is approximately the same as the change in the transmission wavelength caused by the rotation angle of the interference plate.

[0028] The specific implementation principle is as follows:

[0029] Assume that light is vertically incident on the front surface of the prismatic super-high-grade reflective film 5, that is, the initial angle is 0°, the thickness of the piezoelectric module is D, and the cavity length is L1.

[0030] (1) Initial cavity length Available

[0031] (2) When the prism-type super-strong reflective film 5 rotates θ When the cavity length Combined with (1), the relationship between the rotation angle of the prism-type super-strong reflective film 5 and the corresponding cavity mode wavelength can be obtained as follows:

[0032] In the present invention, the working wavelength of the laser diode 1 can be any wavelength. Preferably, from the perspective of easy availability or economical use, a 780nm laser diode can be used. Those skilled in the art can also select laser diodes with other working wavelengths, such as 420nm, 850nm, 1550nm laser diodes, etc.

[0033] In the present invention, in order to realize synchronous rotation of the narrowband interference plate 4 and the prismatic super-strong reflective film 5 bonded together, it can be accomplished by setting a piezoelectric ceramic 7, or installing the narrowband interference plate 4 and the prismatic super-strong reflective film 5 together on a high-precision rotating table 8.

[0034] As a preferred embodiment of the present invention, a piezoelectric ceramic 7 is disposed on the rear surface of the piezoelectric module 6, and the piezoelectric ceramic 7 and the piezoelectric module 6 are used together to realize the change of the rotation angle of the narrow-band interference plate 4 and the prismatic super-strong reflective film 5.

[0035] As an alternative specific implementation method, a high-precision rotating table 8 is used to replace the piezoelectric ceramic 7, and the narrow-band interference plate 4, the prism-type super-strong reflective film 5 and the piezoelectric module 6 are arranged on the high-precision rotating table 8. When the prism-type super-strong reflective film 5 is rotated, the rotation axis is on the rear surface of the piezoelectric module 6 and on the optical path.

[0036] The present invention changes the cavity length by rotating the prism-type super-strong reflective film 5, thereby changing the specific cavity mode frequency of the laser, and simultaneously uses a narrow-band interference plate to assist in adjustment for mode selection, so that the center position of the transmission peak of the interference plate moves with the angle, which is highly synchronized with the change of the specific cavity mode frequency with the angle, thereby ultimately achieving a laser with wide-range frequency tunability. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is a schematic diagram of the structure of a mode-hop-free, large-range continuously tunable external cavity semiconductor laser;

[0038] Figure 2 is a schematic structural diagram of the laser of Example 1;

[0039] Figure 3 is a schematic structural diagram of the laser of Example 2;

[0040] Figure 4 The relationship between the rotation angle and the change of cavity mode frequency of prism-type super-high-grade reflective films of different thicknesses

[0041] Figure 5 The relationship between the rotation angle of the prism-type super-strong reflective film of different thicknesses and the change in the cavity mode wavelength;

[0042] Figure 6 The figure is a comparison of the relationship between the rotation angle of the prism-type super-strong reflective film of different thicknesses and the cavity mode wavelength, and the relationship between the rotation angle of the interference plate and the transmission wavelength.

[0043] Among them, 1. Laser diode, 2. Collimating lens, 3. Polarization beam splitter prism, 4. Narrow-band interference plate, 5. Prism-type super-strong reflective film, 6. Piezoelectric module, 7. Piezoelectric ceramics, 8. High-precision rotating table. DETAILED DESCRIPTION

[0044] The following examples are used to explain the technical solutions of the present invention in a non-limiting manner.

[0045] In the present invention, for the sake of convenience of explanation and not limitation, the direction toward the laser is referred to as "front", and the direction toward the piezoelectric module is referred to as "rear".

[0046] Example 1

[0047] like Figure 1-2 The schematic diagram of the structure of the mode-hop-free, large-range, continuously tunable external cavity semiconductor laser shown in the figure comprises a laser diode 1, a collimating lens 2, a polarization beam splitter prism 3, a narrowband interference plate 4, a prism-type super-strong grade reflective film 5 and a piezoelectric module 6 which are sequentially arranged on the optical path, wherein the narrowband interference plate 4 is bonded to the front surface of the prism-type super-strong grade reflective film 5, and the piezoelectric module 6 is bonded to the rear surface of the prism-type super-strong grade reflective film 5, and the piezoelectric module has a thickness D. When the narrowband interference plate 4, the prism-type super-strong grade reflective film 5 and the piezoelectric module 6 rotate together, the rotation axis is located on the optical path of the laser and coincides with the rear surface of the piezoelectric module 6.

[0048] The initial angle is that the light is incident vertically on the front surface of the prism-type super-strong reflective film 5, at which time the incident angle is 0°, the thickness of the piezoelectric module is D, the cavity length is L1, the working band is λ0, and the working band of the interference film is 780nm. The angle between the incident light and the normal of the narrow-band interference film 4 and the prism-type super-strong reflective film 5 can be adjusted simultaneously by the piezoelectric module in combination with the piezoelectric ceramic 7.

[0049] During operation, the coherent light beam output by the laser diode 1 is collimated into parallel light by the collimating lens 2. The parallel light passes through the narrowband interference plate 4 and the out-of-band light is filtered out to obtain narrowband light. The narrowband light reaches the front surface of the prism-type super-strong reflective film 5 to obtain reflected light in the opposite direction of the narrowband light. The reflected light returns to the laser diode 1 after passing through the narrowband interference plate 4 and the polarization beam splitter prism 3. When the oscillation in the cavity reaches the threshold, the polarization beam splitter prism 3 outputs laser light. The piezoelectric module 6 cooperates with the piezoelectric ceramic 7 to simultaneously adjust the angle between the incident light and the narrowband interference plate 4 and the angle between the incident light and the prism-type super-strong reflective film 5, so as to achieve synchronous changes in the central frequency of the interference plate transmission peak and the cavity mode frequency, and achieve continuous tunability of the output frequency over a wide range.

[0050] By selecting the appropriate thickness of the piezoelectric module 6, the rotating narrow-band interference plate 4 and the prism-type super-strong level reflective film 5 cause the interference plate transmission peak center frequency and the cavity mode frequency to change synchronously. Specifically, when it is assumed that the light is vertically incident on the front surface of the prism-type super-strong level reflective film 5, that is, the initial angle is 0°, the cavity length is 10 cm, and the working band is 780 nm, the piezoelectric modules with different thicknesses are selected, which are D=20 cm, D=22 cm, D=24 cm, D=25 cm and 28 cm respectively.

[0051] Figure 4 The relationship between the rotation angle of piezoelectric modules of different thicknesses and the change in cavity mode frequency obtained by measurement is given, where the horizontal axis is the change in the incident angle of the prism-type super-strong reflective film, and the vertical axis is the change in the cavity mode frequency.

[0052] Figure 5 The relationship between the rotation angle of piezoelectric modules of different thicknesses and the change in cavity mode wavelength obtained by measurement is given, where the horizontal axis is the change in the incident angle of the prism-type super-strong reflective film, and the vertical axis is the change in the cavity mode wavelength.

[0053] Figure 6 The relationship between the rotation angle of the prism-type super-strong reflective film of different thicknesses and the cavity mode wavelength and the relationship between the rotation angle of the interference plate and the transmission wavelength are compared. The horizontal axis is the change in the angle between the incident light and the prism-type super-strong reflective film (interference plate), and the vertical axis is the wavelength.

[0054] from Figure 6 It can be seen that the calculated change in the transmission wavelength Δλ when the interference plate (red solid line) rotates at an angle of 0-14° IF , is 780nm-774.3nm=5.7nm. Figure 5 It can be seen that when the rotation angle is 14°, the cavity mode wavelength change corresponding to the rotation angle of the 24mm piezoelectric module (green dashed line) is about 5.73nm. Figure 6It can also be seen that the relationship curve between the rotation angle of the piezoelectric module with a thickness of 24mm and the cavity mode wavelength (green dotted line) and the relationship curve between the rotation angle of the interference plate and the transmission wavelength Δλ IF They almost completely overlap within 0-14°, that is, the rotating narrow-band interference plate 4 and the prism-type super-strong reflective film 5 cause the changes in the central frequency of the interference plate transmission peak and the cavity mode frequency to be approximately the same.

[0055] Therefore, when the cavity length is 10 cm and the working band is 780 nm, a piezoelectric module with a thickness of 24 mm is selected to achieve simultaneous rotation with the interference plate. As the angle increases / decreases, the transmission peak center frequency of the interference plate and the cavity mode frequency increase / decrease simultaneously, overcoming the continuous frequency modulation problem and achieving continuous tunability of the laser frequency over a large range.

[0056] Example 2

[0057] The same configuration as in Example 1, except that a high-precision rotating table 8 is used to replace the piezoelectric ceramic 7, and the narrow-band interference plate 4, the prism-type super-strong reflective film 5 and the piezoelectric module 6 bonded together are all arranged on the high-precision rotating table 8, and ensure that when rotation occurs, the rotation axis is on the rear surface of the piezoelectric module 6 and on the optical path.

[0058] Similar to Example 1, a piezoelectric module 6 with a thickness of 24 mm is selected, and the angle between the incident light and the interference plate 4, as well as the angle between the incident light and the prismatic super-strong reflective film 5 are adjusted by a high-precision rotating table 7, so that the center position of the transmission peak of the interference plate moves with the angle and the change of the specific cavity mode frequency with the angle is highly synchronized, thereby overcoming the problem of continuous frequency modulation and realizing continuous tunability of the laser frequency over a large range.

[0059] The mode-hopping-free, large-range continuously tunable external cavity semiconductor laser of the present invention can realize large-range continuous tunability of laser frequency. Compared with the prior art, the present invention overcomes the mode-hopping phenomenon caused by the change of laser frequency with cavity length, that is, the change of cavity mode frequency, and innovatively utilizes the sensitivity of laser frequency to the rotation angle of narrow-band interference plate. By simultaneously changing the interference plate angle and cavity length, the center frequency of the interference plate transmission peak is highly synchronized with the change of specific cavity mode frequency with angle, thereby realizing a laser whose laser output frequency does not mode-hop and ensures the continuous tunability of output frequency with cavity mirror rotation angle, solving the problem of continuous frequency modulation, and finally realizing a laser with large-range continuous frequency tunability. The mode-hopping-free, large-range continuously tunable external cavity semiconductor laser of the present invention can meet the demand for large-range tunability of laser frequency, thereby promoting the application and development of laser technology in various fields.

Claims

1. A mode-hop-free, large-range continuously tunable external cavity semiconductor laser, the laser comprising a laser diode (1), a collimating lens (2), a polarization beam splitter prism (3), a narrow-band interference plate (4) and a prism-type super-strong reflective film (5) arranged in sequence on an optical path, characterized in that The laser further comprises a piezoelectric module (6), the narrowband interference plate (4) is bonded to the front surface of the prism-type super-strong grade reflective film (5), the piezoelectric module (6) is bonded to the rear surface of the prism-type super-strong grade reflective film (5), the piezoelectric module (6) has a thickness D, and when the prism-type super-strong grade reflective film (5) rotates, the rotation axis is located on the optical path of the laser and coincides with the rear surface of the piezoelectric module (6); A coherent light beam emitted by a laser diode (1) is collimated into parallel light by a collimating lens (2); the parallel light is passed through a polarization beam splitter prism (3) to obtain horizontal polarized light or vertical polarized light in the same direction as the laser diode (1); the horizontal polarized light or vertical polarized light is incident on a narrow-band interference plate (4) to obtain narrow-band light; the narrow-band light reaches the front surface of a prism-type super-strong reflective film (5) and is reflected and returned to the laser diode (1); when the oscillation in the cavity reaches a threshold value, the polarization beam splitter prism (3) outputs laser light.

2. The mode-hop-free, wide-range continuously tunable external cavity semiconductor laser according to claim 1, characterized in that The thickness D satisfies the following relationship: Dl=Dl IF =λ0-λ′(φ) Moreover, θ=φ Δλ is the change in the cavity mode wavelength when the prism-type super-strong reflective film (5) rotates by θ, Δλ IF is the wavelength change of the center position of the transmission peak of the narrowband interference plate (4) when it rotates by φ, λ ′ (φ) is the wavelength at the center position of the transmission peak of the narrowband interference plate (4) when the narrowband interference plate (4) is rotated by φ, λ0 is the transmission wavelength when the narrowband interference plate (4) is perpendicular to the light beam, λ is the initial wavelength output by the laser diode (1), L1 is the distance from the output light end face of the laser diode (1) to the front surface of the prism-type super-strong level reflective film (5) when the incident angle of the output light of the laser diode (1) to the prism-type super-strong level reflective film (5) is 0°, D is the thickness of the piezoelectric module (6), θ is the incident angle of the prism-type super-strong level reflective film (5), φ is the incident angle of the narrowband interference plate (4), n eff is the refractive index of the narrowband interference plate (4).

3. The mode-hop-free, wide-range continuously tunable external cavity semiconductor laser according to claim 1, characterized in that The thickness D satisfies the following relationship: λ(θ)=λ ′ (φ) and θ=φ, where, λ(θ) is the cavity mode wavelength when the prism-type super-strong reflective film (5) rotates by θ, λ ′ (φ) is the wavelength at the center position of the transmission peak of the narrow-band interference plate when the narrow-band interference plate is rotated by φ, θ is the incident angle of the prism-type super-strong grade reflective film (5), φ is the incident angle of the narrow-band interference plate (4), L1 is the distance from the output light end face of the laser diode (1) to the front surface of the prism-type super-strong grade reflective film (5) when the incident angle of the prism-type super-strong grade reflective film (5) is 0°, D is the thickness of the piezoelectric module (6), λ0 is the transmission wavelength when the narrow-band interference plate (4) is perpendicular to the light beam, and n eff is the refractive index of the narrowband interference plate (4), and n is a natural number.

4. The mode-hop-free, wide-range continuously tunable external cavity semiconductor laser according to claim 1, characterized in that A piezoelectric ceramic (7) is arranged on the rear surface of the piezoelectric module (7).

5. The mode-hop-free, wide-range continuously tunable external cavity semiconductor laser according to claim 4, characterized in that When the piezoelectric ceramic (7) is replaced by a high-precision rotating table (8), a narrow-band interference plate (4), a prism-type super-strong reflective film (5) and a piezoelectric module (6) are arranged on the high-precision rotating table (8), and when the prism-type super-strong reflective film (5) is rotated, the rotation axis is located on the rear surface of the piezoelectric module (6) and on the optical path.

6. The mode-hop-free, wide-range continuously tunable external cavity semiconductor laser according to claim 1, characterized in that The initial wavelength of the laser diode (1) matches the transmission peak center wavelength of the narrow-band interference plate (4).

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