JP7485304B2 - Narrow Linewidth Laser - Google Patents

Description

The present invention relates to a narrow linewidth laser that reduces the spectral linewidth of a semiconductor laser.

Narrow-linewidth lasers are essential devices for realizing digital optical coherent systems that handle the phase information of light as a signal and increase the transmission capacity of optical communications, as well as optical measurement systems with dramatically improved accuracy.

Since the spectral linewidth of a semiconductor laser depends heavily on the resonance characteristics of the laser resonator, attempts have been made to reduce the oscillation spectral linewidth by modifying the resonator structure. Attempts to reduce the spectral linewidth by improving the resonance characteristics through elongation of the laser resonator have been disclosed (Non-Patent Document 1). However, when the resonator is long, problems arise such as distortion being introduced into the element when the laser is mounted, widening the spectral linewidth.

Also, attempts have been disclosed to reduce the lasing spectral linewidth by increasing the cavity length and drawing the oscillating light into its own light, using a semiconductor laser with an external resonator provided by arranging a reflecting mirror, diffraction grating, optical fiber, etc. outside the semiconductor laser (e.g., Non-Patent Documents 2 and 3). However, while this method can reduce the spectral linewidth, it generates frequency noise in a frequency region related to the round-trip time of the light inside the resonator, which has been a problem in application to systems.

Furthermore, attempts have been disclosed to realize a narrow-linewidth semiconductor laser by using a semiconductor optical amplifier as the semiconductor optical amplification medium and coupling an external optical filter as a resonator (for example, Non-Patent Document 4). However, this method lengthens the length of the laser resonator, which makes the light source (laser) larger and makes it difficult to miniaturize it. Thus, there is a trade-off between the oscillation spectral linewidth of a semiconductor laser and the size of the light source.

The optical negative feedback method has been developed as a technology for miniaturizing narrow-linewidth semiconductor lasers, and its effectiveness has been demonstrated (Non-Patent Documents 5, 6). With this technology, a narrow-linewidth semiconductor laser can be constructed using only a single-mode semiconductor laser and an optical filter, making it possible to achieve both a narrow linewidth and a compact light source. The basic principle is that the optical filter converts the frequency fluctuation of the semiconductor laser into optical field amplitude fluctuations, and the light is fed back to the semiconductor laser to induce frequency modulation, which then modulates the oscillation frequency in the opposite direction to achieve negative feedback operation.

H. Ishii, K. Kasaya, and H. Oohashi, "Narrow spectral linewidth operation (<160 kHz) in widely tunable distributed feedback laser array," Electron. Lett. 46(10), 714-715 (2010). S. Saito, O. Nilsson and Y. Yamamoto, "Oscillation center frequency tuning, quantum FM noise, and direct frequency modulation characteristics in external grating loaded semiconductor lasers," IEEE J. Quantum Electronics, vol. QE-18, no. 6, pp. 961-970, June 1982. M. F. Ferreira, J. F. Rocha, and J. L. Pinto, "Noise and modulation performance of Fabry-Perot and DFB semiconductor lasers with arbitrary external optical feedback," IEE Proceedings, vol.137, Pt. J, No. 6, pp. 361-369, 1990. T. Kita, R. Tang, and H. Yamada, "Narrow Spectral Linewidth Silicon Photonic Wavelength Tunable Laser Diode for Digital Coherent Communication System," IEEE J. Sel. Top. Quantum Electron., vol. 22, No. 6, 1500612, 2016. K. Aoyama, N. Yokota, and H. Yasaka, "3-kHz Spectral Linewidth Laser Assembly with Coherent Optical Negative Feedback," IEEE Photonics Technology Letters, vol. 30, No. 3, pp. 277-280, 2018. (Feb.) / DOI 10.1109/LPT.2017.2783365 S. Sato, G. Aizawa, N. Yokota, and H. Yasaka, "Performance Improvement of Optical Negative Feedback Laser by Reducing Feedback Loop Length," IEICE Electronics Express, vol. 17, No. 4, 20190750, 2020. (Feb.) / DOI 10.1587/elex.17.20190750

However, to maximize the narrowing of the oscillation spectral linewidth, it is necessary to improve the coupling efficiency between the single-mode semiconductor laser and the optical filter.

In addition, in the conversion from the oscillation light frequency fluctuation in the optical filter to the feedback light electric field amplitude fluctuation, a multiple interference effect is introduced to improve the frequency discrimination efficiency, but the frequency discrimination efficiency decreases due to the propagation loss of the material that forms the optical filter. Therefore, it is necessary to improve the frequency discrimination function of the optical filter.

In order to solve the above-mentioned problems, the narrow linewidth laser according to the present invention comprises a semiconductor laser, a semiconductor optical amplifier, and an optical filter, and the output light of the semiconductor laser is input to the semiconductor laser as feedback light via the semiconductor optical amplifier and the optical filter, and when the optical oscillation frequency of the semiconductor laser fluctuates, the filter fluctuates the intensity of the feedback light in accordance with the fluctuation of the optical oscillation frequency, the feedback light changes the optical oscillation frequency in the opposite direction to the fluctuation of the optical oscillation frequency, and the semiconductor optical amplifier amplifies the output light of the semiconductor laser and the feedback light.

The present invention provides a small, narrow-linewidth laser that reduces the spectral linewidth of a semiconductor laser.

FIG. 1 is a diagram showing the configuration of a narrow linewidth laser according to a first embodiment of the present invention. FIG. 2 is a diagram for explaining the effect of the narrow linewidth laser according to the first embodiment of the present invention. FIG. 3 is a diagram showing the configuration of a narrow linewidth laser according to a second embodiment of the present invention. FIG. 4 is a diagram for explaining the effect of the narrow linewidth laser according to the second embodiment of the present invention. FIG. 5 is a diagram showing the configuration of a narrow linewidth laser according to a third embodiment of the present invention. FIG. 6 is a diagram for explaining the effect of the narrow linewidth laser according to the third embodiment of the present invention. FIG. 7 is a diagram showing the configuration of a narrow linewidth laser according to a fourth embodiment of the present invention.

First Embodiment
A narrow linewidth laser according to a first embodiment of the present invention will be described with reference to FIGS.

<Configuration of narrow linewidth laser>
As shown in FIG. 1, a narrow linewidth laser 10 according to this embodiment includes a semiconductor laser 1, a first lens 2_1, a second lens 2_2, an optical filter 3, and a semiconductor optical amplifier 4.

In the narrow linewidth laser 10, a semiconductor laser 1, a second lens 2_2, a semiconductor optical amplifier 4, a first lens 2_1, and an optical filter 3 are arranged in this order.

The output light of the semiconductor laser 1 is amplified by the semiconductor optical amplifier 4 via the second lens 2_2, and input to the optical filter 3 via the first lens 2_1. The output light (reflected light) of the optical filter 3 is amplified by the semiconductor optical amplifier 4 via the first lens 2_1, and is fed back (input) to the semiconductor laser 1 via the second lens 2_2.

The semiconductor laser 1 is a distributed feedback (DFB) single-mode semiconductor laser with a resonator length of 350 μm.

The optical filter 3 is a Fabry-Perot etalon made of optical glass with a thickness of 4 mm. The optical filter 3 has reflective films on the opposing end faces, and periodically changes the transmitted light intensity and reflected light intensity according to the optical frequency interval determined by the spacing of the reflective films and the wavelength of the light.

The semiconductor optical amplifier 4 is a semiconductor optical amplifier with an element length of 150 μm.

<Operation of Narrow Linewidth Laser>
The operation of the narrow linewidth laser 10 according to the present embodiment will be described.

First, the negative feedback effect in the operation of the narrow linewidth laser 10 will be described. For example, the operating point of the optical filter 3 in the narrow linewidth laser 10 is set so that the intensity of the reflected light increases as the optical oscillation frequency of the semiconductor laser 1 increases. In this setting, when the optical oscillation frequency of the semiconductor laser 1 increases and the optical oscillation frequency of the light input to the optical filter 3 increases, the intensity of the reflected light increases.

In this way, the intensity of the light (reflected light) returning to the semiconductor laser 1 increases, and the light in the semiconductor gain medium is amplified by stimulated emission. At this time, the carriers (electrons, holes) in the semiconductor gain medium of the semiconductor laser 1 are consumed, and the carrier density decreases.

As a result, the carrier plasma effect increases the refractive index of the semiconductor gain medium and increases the optical effective length of the optical resonator of the semiconductor laser 1, so that the oscillation wavelength increases and the optical oscillation frequency decreases. In this way, an inverse change (decrease) occurs in response to a fluctuation (increase) in the optical oscillation frequency of the semiconductor laser 1. In other words, the fluctuation (increase) in the optical oscillation frequency is suppressed.

Here, the output light of the semiconductor laser 1 is amplified by the semiconductor optical amplifier 4 and input to the optical filter 3. As the intensity of the input light increases, the intensity of the reflected light of the optical filter 3 also increases.

In addition, the light returning from the optical filter 3 to the semiconductor laser 1 (feedback light) is amplified by the semiconductor optical amplifier 4.

In this way, the semiconductor optical amplifier 4 compensates for the coupling loss between the semiconductor laser 1 and the optical filter 3, improving the coupling efficiency.

As a result, compared to conventional narrow-linewidth lasers, the carrier-plasma effect increases the opposite change (e.g., decrease) that occurs in response to a fluctuation (e.g., increase) in the optical oscillation frequency of the semiconductor laser 1. Therefore, the fluctuation (increase) in the optical oscillation frequency can be suppressed more effectively than in conventional narrow-linewidth lasers.

On the other hand, when the optical oscillation frequency of the semiconductor laser 1 decreases, the optical oscillation frequency increases due to the opposite effect to that described above, and fluctuations (decrease) in the optical oscillation frequency are suppressed more effectively than in conventional narrow linewidth lasers.

In this way, the light returning from the optical filter through the semiconductor optical amplifier changes the optical oscillation frequency in the semiconductor laser in the opposite direction to the fluctuation of the optical oscillation frequency.

As described above, when the optical oscillation frequency of the semiconductor laser 1 fluctuates, a negative optical feedback effect occurs due to the reflected light from the optical filter 3, and the fluctuation of the optical oscillation frequency is more effectively suppressed. Therefore, the spectrum of the semiconductor laser 1 can be more effectively narrowed in linewidth.

<Effects of narrow linewidth laser>
FIG. 2 shows the dependence of the oscillation spectral linewidth of the narrow linewidth laser 10 on the bias current to the semiconductor optical amplifier 4. As shown in FIG.

For comparison, the oscillation spectrum linewidth of the semiconductor laser 1 alone is 10 MHz. In addition, the oscillation spectrum linewidth of a conventional structure consisting of a semiconductor laser, a lens, and an optical filter (a structure without a semiconductor amplifier) is 3 kHz.

As can be seen from FIG. 2, in the narrow linewidth laser 10, when the bias current is 0 mA, the oscillation spectral linewidth is 10 MHz. At this time, since the light does not pass through the semiconductor optical amplifier 4, this corresponds to the oscillation spectral linewidth of the semiconductor laser 1 alone.

Increasing the bias current to 6mA or more reduces the spectral linewidth, and at a bias current of 13mA it can be reduced to below 500Hz.

Here, the optical coupling efficiency between the semiconductor laser 1 and the Fabry-Perot etalon 3 is about 0.4 in the conventional structure, but is increased to 1 or more in the narrow linewidth laser 10.

In this way, the narrow linewidth laser according to this embodiment can improve the optical coupling between the semiconductor laser and the optical filter, and can more effectively narrow the linewidth of the semiconductor laser spectrum.

Second Embodiment
A narrow linewidth laser according to a second embodiment of the present invention will be described with reference to FIGS.

<Configuration of narrow linewidth laser>
As shown in FIG. 3, the narrow linewidth laser 20 according to this embodiment includes a semiconductor laser 1, a first lens 2_1, an optical filter 3, and a semiconductor optical amplifier 4.

In the narrow linewidth laser 20, the semiconductor laser 1 and the semiconductor optical amplifier 4 are monolithically integrated, and the first lens 2_1 and the optical filter 3 are arranged in that order.

Thus, the narrow linewidth laser 20 differs from the first embodiment in that the semiconductor laser 1 and the semiconductor optical amplifier 4 are monolithically integrated and do not include the second lens 2_2, but is otherwise substantially similar.

In addition, the narrow linewidth laser 20 according to this embodiment operates in the same manner as the first embodiment.

<Effects of narrow linewidth laser>
FIG. 4 shows the dependence of the oscillation spectral linewidth of the narrow linewidth laser 20 on the bias current to the semiconductor optical amplifier 4. As shown in FIG.

The spectral linewidth is 10 MHz when the bias current is 0 mA. Increasing the bias current to 2 mA or more reduces the spectral linewidth, and can be reduced to around 500 Hz when the bias current is 9 mA.

In this way, the narrow linewidth laser according to this embodiment can improve the optical coupling between the semiconductor laser and the optical filter, and can more effectively narrow the linewidth of the semiconductor laser spectrum.

Third Embodiment
A narrow linewidth laser according to a third embodiment of the present invention will be described with reference to FIGS.

<Configuration of narrow linewidth laser>
5, a narrow linewidth laser 30 according to the present embodiment includes a semiconductor laser 1 and an optical filter 31. The semiconductor laser 1 and the optical filter 31 are hybrid-integrated.

The semiconductor laser 1 is a distributed feedback (DFB) single-mode semiconductor laser with a resonator length of 350 μm.

The optical filter 31 includes an input/output optical waveguide 32, a loop waveguide 33, a ring resonator 34 coupled to the loop waveguide 33, a first semiconductor optical amplifier 4_1, and a second semiconductor optical amplifier 4_2.

The optical filter 31 uses, for example, a Si waveguide consisting of a Si core and SiO2 cladding. The Si waveguide allows for sharp bending due to the large difference in refractive index between the core and cladding, making it suitable for small size and high integration. The width of the waveguide structure used in the optical filter 31 is 400 nm, and the layer thickness of the waveguide core is 220 nm.

The loop waveguide 33 in the optical filter 31 may have a configuration in which the light incident from the semiconductor laser 1 is coupled with the ring resonator 34 and fed back to the semiconductor laser 1, and the top surface shape of the configuration may be a polygon with curved corners. Alternatively, the top surface shape may be a circle or an ellipse. If the top surface shape of the loop waveguide 33 is circular, the radius is about 20 μm.

The ring resonator 34 in the optical filter 31 may have any structure in which the light coupled from the loop waveguide 33 circulates, and may have a circular top surface shape as shown in FIG. 5. Alternatively, the top surface shape may be an ellipse or a polygon. If the top surface shape of the ring resonator 34 is circular, the radius of the ring resonator 34 is about 210 μm.

The gap between the loop waveguide 33 and the ring resonator 34 is 50 μm, and the length is sufficient to allow optical coupling.

The semiconductor laser 1 and the optical filter 31 are butt-coupled.

The first semiconductor optical amplifier 4_1 is integrated into the ring resonator 34 to improve the characteristics of the optical filter 31 (described later). The element length of the first semiconductor optical amplifier 4_1 is 100 μm.

The second semiconductor optical amplifier 4_2 is integrated into the input/output optical waveguide 32 to compensate for the coupling loss caused by this butt coupling. The element length of the second semiconductor optical amplifier 4_2 is 100 μm.

The light input from the semiconductor laser 1 to the optical filter 31 is coupled to the Si ring optical waveguide 11, undergoes multiple interference, and then is coupled again to the Si input/output optical waveguide 12 and fed back to the single mode semiconductor laser 1.

The operating principle of the narrow linewidth laser 30 in this embodiment is the same as in the first embodiment.

For example, when the oscillation frequency of the semiconductor laser 1 increases, the ring resonator 34 changes the amplitude to an increase, increasing the amplitude of the feedback light (reflected light). When the feedback light (reflected light) increases, the carrier density of the semiconductor laser 1 decreases, and the optical oscillation frequency decreases due to the carrier plasma effect, suppressing fluctuations (increases) in the optical oscillation frequency.

Here, the first semiconductor optical amplifier 4_1 reduces the propagation loss in the ring resonator 34, improving the characteristics of the optical filter 31 and increasing the electric field amplitude of the feedback light.

In addition, the second semiconductor optical amplifier 4_2 improves the coupling efficiency between the semiconductor laser 1 and the optical filter 31. As a result, the feedback light to the semiconductor laser 1 is amplified.

In this way, when the optical oscillation frequency of the semiconductor laser 1 fluctuates, a negative optical feedback effect occurs due to the reflected light from the optical filter 31, and the fluctuation of the optical oscillation frequency is more effectively suppressed. Therefore, the spectrum of the semiconductor laser 1 can be more effectively narrowed in linewidth.

<Effects of narrow linewidth laser>
6 shows the propagation loss dependence of the frequency discrimination characteristic of the optical filter 31 in the narrow linewidth laser 30. Here, the frequency discrimination characteristic is a characteristic in which the electric field reflection coefficient (the amplitude of the feedback optical field) changes with the change in optical frequency. The free spectral range (FSR) is 50 GHz.

The frequency discrimination efficiency decreases as the amount of propagation loss increases. Therefore, by reducing the propagation loss of the ring optical waveguide 11, the frequency discrimination efficiency can be increased, i.e., the electric field amplitude of the feedback light can be increased, thereby reducing the oscillation spectral linewidth.

By placing the first semiconductor optical amplifier 4_1 in the ring resonator 34, this propagation loss can be compensated for. Here, the light coupled to the ring resonator 34 without the semiconductor optical amplifier undergoes multiple interference in the Si ring optical waveguide, but is attenuated due to the waveguide propagation loss, and the filter characteristics deteriorate. Here, the propagation loss of the optical waveguide is about 0.83 [1/cm]. On the other hand, the propagation loss of the ring resonator 34 with the first semiconductor optical amplifier 4_1 is 0.05 [1/cm].

As can be seen from FIG. 6, by using the first semiconductor optical amplifier 4_1 to reduce the propagation loss from 0.83 [1/cm] to 0.05 [1/cm], the frequency discrimination efficiency can be increased by about 16 times, from 0.2 [1/GHz] to 3.8 [1/GHz].

In this way, the first semiconductor optical amplifier 4_1 can reduce the propagation loss in the ring resonator 34 and improve the characteristics of the optical filter 31.

As a result, in addition to the improvement in the characteristics of the optical filter 31 by the first semiconductor optical amplifier 4_1, the coupling efficiency between the semiconductor laser 1 and the optical filter 31 by the second semiconductor optical amplifier 4_2 is improved, so that the oscillation spectral linewidth of the semiconductor laser 1 can be reduced. The oscillation spectral linewidth of a conventional semiconductor laser (single mode semiconductor laser 1 alone) is 10 MHz. On the other hand, the narrow linewidth laser 30 can reduce the oscillation spectral linewidth to 1 kHz.

The narrow linewidth laser according to this embodiment can more effectively narrow the spectrum of the semiconductor laser, and the element can be made smaller than the narrow linewidth lasers according to the first and second embodiments.

In this embodiment, an example is shown in which the second semiconductor optical amplifier is disposed in the input/output optical waveguide, but the second semiconductor optical amplifier may also be disposed in the loop waveguide.

<Fourth embodiment>
A narrow linewidth laser according to a fourth embodiment of the present invention will be described with reference to FIG.

<Configuration of narrow linewidth laser>
7, a narrow linewidth laser 40 according to this embodiment includes a semiconductor laser 1, a semiconductor optical amplifier 4, and an optical filter 41. The semiconductor laser 1, the semiconductor optical amplifier 4, and the optical filter 41 are hybrid-integrated.

The semiconductor laser 1 is a distributed feedback (DFB) single-mode semiconductor laser with a resonator length of 350 μm.

The optical filter 41 includes an input/output optical waveguide 32, a loop waveguide 33, a ring resonator 34 coupled to the loop waveguide 33, and a first semiconductor optical amplifier 4_1. The optical filter 41 differs from the third embodiment in that it does not include a second semiconductor optical amplifier 4_2, but is otherwise similar in configuration.

The operating principle of the narrow linewidth laser 40 in this embodiment is substantially the same as that in the third embodiment.

In addition, in the narrow linewidth laser 40, by placing a semiconductor optical amplifier 4 instead of the second semiconductor optical amplifier 4_2 in the third embodiment, the same effect as in the third embodiment is achieved.

The narrow linewidth laser according to this embodiment can more effectively narrow the linewidth of the semiconductor laser spectrum, and the element can be made smaller than the narrow linewidth lasers according to the first and second embodiments.

In the third and fourth embodiments, examples using a first semiconductor optical amplifier and a second semiconductor optical amplifier are shown, but even if either the first or second semiconductor optical amplifier is used, the fluctuation of the optical oscillation frequency of the semiconductor laser can be suppressed and the linewidth of the spectrum of the semiconductor laser can be effectively narrowed.

In addition, multiple semiconductor optical amplifiers may be arranged in at least one of the input/output optical waveguide, the loop waveguide, and the ring resonator in the optical filter.

In the third and fourth embodiments, the material constituting the optical waveguide is Si, but it is self-evident that it can be constructed using semiconductor materials such as SiN, InP, and GaAs. It is also self-evident that this light source can be realized by using InP as the material constituting the optical waveguide and monolithically integrating it with a single-mode semiconductor laser made of an InP-based compound semiconductor.

In the third and fourth embodiments, an example was shown in which the input/output optical waveguide, loop waveguide, ring resonator, and semiconductor optical amplifier in the optical filter are integrated on the same substrate using Si as the substrate, but the substrate is not limited to Si and may be a semiconductor substrate such as InP or GaAs, or a glass substrate. If an InP substrate is used, the configuration of the optical filter described above can be monolithically integrated on the same substrate, and a semiconductor laser can also be monolithically integrated.

In the embodiment of the present invention, an example is shown in which a distributed feedback (DFB) laser is used as the semiconductor laser, but this is not limiting, and a distributed bragg reflector (DBR) laser or the like may also be used, as long as it is a single mode semiconductor laser. InP-based compound semiconductors can be used as the material, and other materials such as InP-based and GaN-based compound semiconductors may also be used depending on the oscillation wavelength.

In an embodiment of the present invention, an InP-based compound semiconductor can be used as the material for the semiconductor optical amplifier, and other materials such as InP-based and GaN-based compound semiconductors may also be used depending on the oscillation wavelength. Rare earth-doped Si can also be used.

In the embodiment of the present invention, examples of the structure, dimensions, materials, etc. of each component in the configuration of the narrow linewidth laser are shown, but the present invention is not limited to these. Anything that can exert the function and effect of the narrow linewidth laser can be used.

The present invention can be applied to optical communication systems, digital optical coherent systems, and optical measurement systems.

1 Semiconductor laser 2_1, 2_2 Lens 3 Optical filter 4 Semiconductor optical amplifier 10 Narrow linewidth laser

Claims (7)

A semiconductor laser;
A semiconductor optical amplifier;
An optical filter;
output light of the semiconductor laser is input to the semiconductor laser as feedback light via the semiconductor optical amplifier and the optical filter;
when the optical oscillation frequency of the semiconductor laser fluctuates, the optical filter fluctuates the intensity of the feedback light in accordance with the fluctuation of the optical oscillation frequency;
the feedback light changes the optical oscillation frequency in a direction opposite to the fluctuation of the optical oscillation frequency;
a semiconductor optical amplifier for amplifying an output light of the semiconductor laser and a feedback light, a lens is provided between the semiconductor optical amplifier and the optical filter;
2. The narrow linewidth laser of claim 1, wherein the optical filter is a Fabry-Perot etalon. 2. The narrow linewidth laser according to claim 1, wherein the optical filter comprises: an input/output optical waveguide; a loop waveguide connected to the input/output optical waveguide; and a ring resonator coupled to the loop waveguide. 4. The narrow linewidth laser according to claim 3, wherein the semiconductor optical amplifier is provided in at least one of the input/output optical waveguide, the loop waveguide, and the ring resonator in the optical filter. the semiconductor optical amplifier disposed in at least one of the input/output optical waveguide and the loop waveguide in the optical filter;
and another semiconductor optical amplifier disposed in the ring resonator in the optical filter. the semiconductor optical amplifier disposed between the semiconductor laser and the optical filter;
and another semiconductor optical amplifier disposed in the ring resonator in the optical filter. 7. The narrow linewidth laser according to claim 3, wherein the optical filter is formed on the same substrate.

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