JP5402110B2 - Light source for optical communication - Google Patents

Description

  The present invention relates to an optical communication system, and more particularly to an optical communication light source that is an optical functional element having a function of converting an electrical signal into an optical signal.

  An optical communication light source for converting an electrical signal into an optical signal is a key component that has played an extremely important role in recent rapid increase in communication traffic. As a technique for realizing a light source for optical communication, a direct modulation system of a semiconductor laser and a system combining a semiconductor laser and an external optical modulator have been mainly employed.

In particular, the direct modulation method of a semiconductor laser is one of the communication methods that has been studied from the earliest as a low-cost and simple method. However, the distance that can be transmitted due to chirping that occurs at the time of modulation is shorter than other systems, and is limited to a transmission distance of about 200 km at a modulation speed of 2.5 Gb / s and about 10 km at a modulation speed of 10 Gb / s. There was a problem of being done.
On the other hand, in a method in which an external modulator such as an electro-absorption modulator using a compound semiconductor material or a Mach-Zehnder modulator using a LiNbO ₃ material is combined with a semiconductor laser, not only the size of chirping but also the code to some extent It is possible to control, and the transmission distance can be greatly increased.

As an example of an electroabsorption modulator made of a compound semiconductor material, a modulator integrated light source made of InP material is disclosed in Non-Patent Document 1, and has been put to practical use as a light source for long-distance optical communication. Further, as an example of a Mach-Zehnder modulator made of a LiNbO ₃ material, Non-Patent Document 2 discloses a particularly miniaturized one.
In both the electroabsorption modulator and the Mach-Zehnder modulator, transmission distances of 600 km to 800 km at a modulation speed of 2.5 Gb / s and 40 km to 80 km at a modulation speed of 10 Gb / s are realized.

  On the other hand, recently, Patent Document 1, Patent Document 2, and Non-Patent Document 3 disclose methods that can further increase the transmission distance. This method is called CML (Charp Managed Laser). By small-signal modulation of the injection current of the semiconductor laser, frequency modulation (FM modulation) is generated in the oscillation wavelength, and the frequency-modulated light is converted into a narrow-band wavelength. By inputting to a filter, it is converted into amplitude-modulated (AM-modulated) light, and an optical signal is transmitted. In a transmission experiment using this CML method, a transmission distance of 250 km is realized at 10 Gb / s.

  The CML system requires a wavelength filter having a large FSR (Free Spectrum Range). Normally, an FSR of about 2 to 3 times the transmission speed is required, so when considering a higher speed signal transmission, for example, a transmission speed of 40 Gb / s or 100 Gb / s, it has an FSR of about 300 GHz. A wavelength filter is required. As the wavelength filter, a filter using a ring resonator or a resonator using a thin film mirror is conceivable. However, no CML device applicable to the high-speed transmission has been reported. Considering loss and manufacturing tolerance, it is not always easy to create a wavelength filter with a large FSR.

JP 2007-139888 A JP 2006-516075 gazette

M.ISHIZAKA et al., IEEE PHOTONICS TECHNOLOGY LETTERS, 1997, VOL.9, NO.10, p.1406 M.SUGIYAMA et al., 30th Eouropean Conference On Optical Communication, 2004, Post-Deadline Session3, Th4.3.5 D. Mahgerefte et al., ELECTRONICS LETTERS, 28th, April, 2005, Vol. 41, No. 9, p.543

  Advances in optical communication technology are being carried out for the purpose of transmitting more information to a greater distance. The expansion of the amount of information is progressing by improving the communication speed and adopting the wavelength division multiplexing transmission system, and the expansion of the transmission distance is beginning to show a tendency to accelerate by introducing CML or the like that surpasses the external modulation system. However, even in the CML systems disclosed in Patent Document 1, Patent Document 2, and Non-Patent Document 3, high-speed modulation such as 40 Gb / s or 100 Gb / s has not been put into practical use. The main factor is the size of the FSR of the wavelength filter. In the CML system, an FSR having a band that is approximately two to three times the modulation band is required, and a filter having a steep transmittance spectrum is required in terms of modulation efficiency at the time of FM-AM conversion. A method for easily and inexpensively manufacturing a wavelength filter using a ring resonator or a resonator using a thin film mirror and having particularly low light transmission loss has not been realized.

  The present invention has been made to solve the above problems, and an object thereof is to provide a light source for optical communication capable of high-speed modulation and long-distance transmission without enlarging the FSR of the wavelength filter.

The light source for optical communication of the present invention includes a laser light source capable of outputting a light wave of a specific wavelength, and an external wavelength filter to which output light from the laser light source is input and the output light intensity changes according to the input wavelength. The external wavelength filter has a configuration in which first and second wavelength filters having different wavelength periods of power transmittance are connected in cascade, and the first and second wavelength filters have first resonance periods having different wavelength periods. The first optical resonator includes a first air gap mirror formed in an optical waveguide optically coupled to the laser light source, and the first air gap mirror. A second air gap mirror formed at a location on the optical waveguide on the output side, and an optical waveguide between the first and second air gap mirrors, and the second optical resonator includes: Said second air A cap mirror, a third air gap mirror formed on the optical waveguide on the output side of the second air gap mirror, and an optical waveguide between the second and third air gap mirrors. The transmittance of the second air gap mirror is set to be lower than the transmittance of the first and third air gap mirrors, and the length of the first and second optical resonators is changed. The first and second optical resonators are set to have different wavelength resonance periods, and the light wave frequency-modulated in the laser light source is converted into intensity modulation by the external wavelength filter.

The light source for optical communication according to the present invention converts the light wave of a frequency-modulated laser light source into intensity modulation by passing through an external wavelength filter having a steep spectrum, and can reduce waveform distortion after transmission. It is a light source for optical communication suitable for long-distance transmission.
The first effect of the light source for optical communication according to the present invention is a method of converting frequency-modulated laser oscillation light into intensity modulation, and therefore, compared with a direct modulation method of a semiconductor laser or a communication method using an external modulator. Long-distance transmission is possible.
The second effect of the light source for optical communication of the present invention is that the small signal frequency modulation of the laser light source is converted into intensity modulation using the wavelength dependency (frequency dependency) of the external wavelength filter. By making the wavelength characteristic steep, intensity modulation with a large extinction ratio can be realized with relatively small frequency modulation, and power consumption of the drive electric circuit can be reduced.
The third effect of the light source for optical communication according to the present invention is that the external wavelength filter is composed of two wavelength filters having different FSRs, thereby making it possible to cause a sufficient difference in transmittance between adjacent transmission peaks. Therefore, the modulation speed can be increased without increasing the FSR of the wavelength filter. That is, since a high-band wavelength filter is unnecessary, the manufacturing cost can be reduced.
The fourth effect of the light source for optical communication according to the present invention is that the external wavelength filter can be constituted by a planar optical circuit (PLC), can be hybrid-integrated with a laser light source, and a semiconductor material can be used as a planar circuit. By using it, monolithic integration is possible, so that downsizing and reduction in mounting cost can be achieved.

It is a schematic diagram which shows the outline of the light source for optical communications which concerns on embodiment of this invention. It is a schematic diagram which shows the structure of the air gap mirror in the waveguide type resonator end surface of the light source for optical communications which concerns on embodiment of this invention. It is a figure which shows the wavelength dependence of the power transmittance of the air gap mirror of FIG. It is a figure which shows the power transmittance | permeability spectrum of the external wavelength filter used for the light source for optical communications which concerns on embodiment of this invention. It is a figure which shows the relationship between the power transmittance | permeability spectrum of the external wavelength filter used for the light source for optical communications which concerns on embodiment of this invention, and the spectrum of NRZ modulation.

Next, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic diagram showing an outline of a light source for optical communication according to an embodiment of the present invention.
In the light source for optical communication shown in FIG. 1, a first silicon optical waveguide 3 having silicon as a core layer on the same upper surface of an SOI (Silicon on insulator) substrate 1 having a structure in which a silicon oxide film is sandwiched between silicon materials and Two silicon optical waveguides 4 are arranged in tandem on the same straight line.

  The first silicon optical waveguide 3 has a tapered silicon tapered optical waveguide 2 at one end thereof. By this silicon tapered optical waveguide 2, the first silicon optical waveguide 3 is optically coupled with low loss to the silicon nitride optical waveguide 8 having the silicon nitride film as a core layer while gradually increasing the spot size of the guided light. The semiconductor laser 10 is optically connected via an optical isolator 9. A non-reflective coating film 5 is applied to one end of the second silicon optical waveguide 4.

  The first silicon optical waveguide 3 and the second silicon optical waveguide 4 are connected via an air gap mirror 13. The first silicon optical waveguide 3, the second silicon optical waveguide 4, and the air gap mirrors 12, 13, and 14 form a structure in which two waveguide optical resonators are connected in cascade. That is, the air gap mirrors 12 and 13 and the first silicon optical waveguide 3 between the air gap mirrors 12 and 13 constitute a first waveguide type optical resonator 15, and the air gap mirrors 13 and 14 and the air gap mirror The second silicon optical waveguide 4 between 13 and 14 constitutes a second waveguide type optical resonator 16. Each of the first waveguide type optical resonator 15 and the second waveguide type optical resonator 16 constitutes a wavelength filter.

  A heater 6 is arranged above the first silicon optical waveguide 3 constituting the first waveguide type optical resonator 15 so that the wavelength transmission characteristic can be changed. Similarly, the second waveguide type is used. A heater 6 is also disposed on the second silicon optical waveguide 4 constituting the optical resonator 16.

2A is a schematic diagram showing the configuration of the air gap mirror 12, FIG. 2B is a schematic diagram showing the configuration of the air gap mirror 13, and FIG. 2C is a schematic diagram showing the configuration of the air gap mirror 14. It is.
The air gap mirror 12 is formed with air gaps 202 and 203 having a width of 150 nm (left and right direction in FIGS. 1 and 2A), and a waveguide length (see FIGS. 1 and 2A) between the air gaps 202 and 203. A silicon optical waveguide 201 of 120 nm is formed.

The air gap mirror 13 is formed with air gaps 207, 209, 210 having a width of 150 nm (left and right direction in FIGS. 1 and 2B), and the waveguide length (see FIGS. 1, 2B) between the air gaps 207, 210. B) Left-right direction) A 120 nm silicon optical waveguide 208 is formed, and a silicon optical waveguide 211 having a waveguide length of 120 nm is formed between the air gaps 209 and 210.
The air gap mirror 14 is formed with air gaps 204 and 206 having a width of 150 nm (left and right direction in FIGS. 1 and 2C), and a waveguide length between the air gaps 204 and 206 (FIGS. 1 and 2C). A silicon optical waveguide 205 with a thickness of 120 nm is formed.

The air gap mirrors 12 and 14 have the same structure, and the air gap mirror 13 has a structure in which the number of periods is larger than that of the air gap mirrors 12 and 14 by one period.
FIG. 3 is a diagram showing the wavelength dependence of the power transmittance of the air gap mirrors 12, 13, and 14. FIG. 3 shows the transmittance obtained by calculation. In the figure, 301 indicates the transmittance of the air gap mirrors 12 and 14, and 302 indicates the transmittance of the air gap mirror 13. According to FIG. 3, it can be seen that the air gap mirrors 12, 13, and 14 all exhibit relatively uniform characteristics with respect to the wavelength.

  The length of the first waveguide type optical resonator 15 (the distance between the air gap mirrors 12 and 13, L1 in FIG. 1) and the length of the second waveguide type optical resonator 16 (the air gap mirror 13). , 14 is different from L2) in FIG. Thus, the wavelength resonance period of the first waveguide optical resonator 15 and the wavelength resonance period of the second waveguide optical resonator 16 are set to be different.

  The operation mechanism of the light source for optical communication according to the present embodiment will be described in detail below. By supplying an injection current that has been subjected to small signal modulation in accordance with an electrical signal from a laser driver (not shown) to the semiconductor laser 10, the semiconductor laser 10 outputs laser light that has been frequency-modulated. This frequency-modulated laser output light passes through the optical isolator 9 and then propagates through the silicon nitride optical waveguide 8, where the spot size of the field is converted by the silicon tapered optical waveguide 2 which is a spot size conversion unit, and low loss is achieved. Is introduced into the first silicon optical waveguide 3.

  The laser output light incident on the first silicon optical waveguide 3 passes through the first waveguide type optical resonator 15 including the air gap mirrors 12 and 13 and the first silicon optical waveguide 3, and further, the air gap mirror. The light is incident on a second waveguide type optical resonator 16 composed of 13 and 14 and the second silicon optical waveguide 4. The laser output light that has passed through the second waveguide type optical resonator 16 is emitted to the outside in a state in which reflection at the end face of the waveguide is sufficiently suppressed by the non-reflective coating film 5. Thus, the electrical signal is converted into an optical signal by the light source for optical communication.

  FIGS. 4A and 4B are diagrams showing power transmittance spectra of a composite resonator (external wavelength filter) including two waveguide type optical resonators 15 and 16. 4A and 4B, the horizontal axis indicates the amount of wavelength transition from the center peak wavelength of wavelength λ (the wavelength at which the resonance peak wavelengths of the two waveguide optical resonators 15 and 16 match) λc. Show. Since there is a difference in FSR between the two waveguide type optical resonators 15 and 16, as is clear from FIG. 4A, the peak of the transmitted light intensity decreases as the wavelength λ of the light moves away from the central peak wavelength λc. It has a wavelength spectrum.

FIG. 4B is an enlarged view of the vicinity of the center peak wavelength λc (λ−λc = 0 in FIG. 4A) of FIG. The laser oscillation light in which the wavelength λ is changed (that is, the frequency is changed) by 402 in FIG. 4B with respect to the center peak wavelength λc is obtained by an external wavelength filter including the waveguide type optical resonators 15 and 16. The transmittance is changed by 401 in FIG. 4B, that is, converted into a light wave whose intensity has changed.
Light that does not pass through the external wavelength filter becomes reflected return light, but is blocked by the isolator 9, so that the reflected return light does not enter the semiconductor laser 10 back.

  Further, in order to convert the light frequency change into the intensity change using the external wavelength filter composed of the waveguide type optical resonators 15 and 16, the center of the laser light wavelength change and the center of the transmittance slope of the external wavelength filter are used. Must be adjusted to an appropriate wavelength position (ie, the centers are matched). Such wavelength position adjustment is performed by the heater 6. That is, when current is supplied to the heater 6 from a heater power source (not shown) and the silicon optical waveguides 3 and 4 are heated by the heater 6, the refractive index of the waveguide layers of the waveguide type optical resonators 15 and 16 is increased by the thermo-optic effect. Since it changes, the peak position of the transmittance spectrum of the external wavelength filter can be shifted along the wavelength axis.

  FIGS. 5A and 5B are diagrams showing the effects of the light source for optical communication according to the present embodiment, and the power transmittance spectrum of the external wavelength filter and the NRZ (Non Return) input to the external wavelength filter. It is a figure which shows the relationship with the wavelength spectrum of the modulated signal light.

  FIG. 5A shows the power transmittance spectrum 502 of the external wavelength filter and the spectrum 501 of the signal light NRZ-modulated at 50 GHz when the waveguide type optical resonators 15 and 16 having an FSR of about 50 GHz are used. ing. As described above, there is a difference in FSR between the two waveguide type optical resonators 15 and 16. Here, the harmonic component of the NRZ modulation away from the center peak wavelength λc (λ−λc = 0 in FIG. 5A) is affected by periodic peaks other than those near the center peak wavelength of the transmittance spectrum of the external wavelength filter. In order not to receive it, the intensity of the transmittance spectrum of the external wavelength filter is set to attenuate as the wavelength λ becomes farther from the center peak wavelength λc.

FIG. 5B shows the power transmittance spectrum 602 of the external wavelength filter and the spectrum of the signal light NRZ-modulated at 50 GHz when the FSRs of the waveguide type optical resonators 15 and 16 are set to the same value (50 GHz). 501. In this case, the transmittance spectrum of the external wavelength filter shows the same spectral intensity as that in the vicinity of the center peak wavelength even in the wavelength band away from the center peak wavelength λc, and affects the harmonic spectrum of NRZ modulation It will be.
A prominent feature of this embodiment is that the center peak of the transmittance spectrum of the external wavelength filter comprising the waveguide type optical resonators 15 and 16 is made different by making the FSRs of the two waveguide type optical resonators 15 and 16 different. The purpose is to suppress the transmittance peak intensity other than the wavelength.

  As described above, the light source for optical communication according to the present embodiment performs frequency modulation corresponding to transmission data on the oscillation light output from the laser light source (semiconductor laser 10), and the frequency modulated light is externally filtered. Is converted into amplitude-modulated light (light intensity modulation data) and output, and light can be transmitted over a long distance compared to a direct modulation method of a semiconductor laser or a transmission method using an external modulator.

  The frequency-modulated laser light output from the semiconductor laser 10 passes through the optical isolator 9 and is input to two wavelength filters (waveguide optical resonators 15 and 16) having different transmittances FSR. Since some of the transmittance peaks of the two wavelength filters are set to coincide with the input laser light wavelength, the frequency modulation of the laser light (that is, modulation of the oscillation wavelength) is performed by the two wavelength filters. Converted to light intensity modulation.

  When a single FSR wavelength filter is used as the wavelength filter, the speed of frequency modulation is limited to a fraction of the FSR. The reason for this limitation is that intensity modulation is disturbed when the tail of the distribution of the light wave spectrum (wavelength spectrum or frequency spectrum) during modulation is applied to a transmission peak in a wavelength region separated by FSR. Therefore, as the modulation rate increases, a wavelength filter with a larger FSR is required.

  On the other hand, since the external wavelength filter of the present embodiment is composed of two wavelength filters having different FSRs, from the maximum value of the transmission peak (the point where the transmittance peaks of the two wavelength filters match). The transmission peak in the wavelength region separated by the FSR becomes smaller because the transmittance peaks of the two wavelength filters are shifted. That is, in this embodiment, even if the modulation speed is increased without increasing the FSR of the external wavelength filter, the intensity modulation disturbance can be suppressed.

[Second Embodiment]
In the first embodiment, a silicon optical waveguide is used as the external wavelength filter. However, the external wavelength filter is not limited to this, and the external wavelength filter is a compound semiconductor material optical waveguide such as InP or GaAs. Also good.

  In the first embodiment, the heater 6 is used to adjust the resonance peak positions of the two waveguide type optical resonators 15 and 16, but the Franz Keldisch effect and the quantum well structure by the compound semiconductor are used. It is also possible to use an electro-optic effect such as the quantum confined Stark effect used. In this case, an electrode for applying voltage or injecting current to the silicon optical waveguides 3 and 4 constituting the two waveguide optical resonators 15 and 16 is disposed in the vicinity of the silicon optical waveguides 3 and 4. What is necessary is just to adjust the electric power supplied from a power supply to an electrode so that the resonance peak wavelength of the two waveguide type optical resonators 15 and 16 may become a desired position.

It is also possible to use a quartz-based material that has been used frequently as a material for the external wavelength filter, and is particularly effective in an element configuration in which the element size is increased but the optical loss needs to be reduced.
Further, the semiconductor laser and the external wavelength filter may be hybrid-integrated on the same substrate.

  Further, when an external wavelength filter made of an optical waveguide of a compound semiconductor material such as InP or GaAs is used, the semiconductor laser and the external wavelength filter may be monolithically integrated on the same substrate. Further, in this case, the active layer of the semiconductor laser and the waveguide core layer of the external wavelength filter are each configured with a multiple quantum well structure having different band gaps, and the band gap of the active layer of the semiconductor laser is set to the waveguide of the external wavelength filter. You may make it set smaller than the band gap of a core layer.

  The present invention can be applied to a light source for optical communication in an optical communication system.

  DESCRIPTION OF SYMBOLS 1 ... SOI substrate, 2 ... Silicon taper optical waveguide, 3 ... 1st silicon optical waveguide, 4 ... 2nd silicon optical waveguide, 5 ... Antireflection coating film, 6 ... Heater, 8 ... Silicon nitride film optical waveguide, 9 DESCRIPTION OF SYMBOLS ... Optical isolator, 10 ... Semiconductor laser, 12, 13, 14 ... Air gap mirror, 15 ... 1st waveguide type optical resonator, 16 ... 2nd waveguide type optical resonator, 201, 205, 208, 211 ... Silicon optical waveguide, 202, 203, 204, 206, 207, 209, 210 ... Air gap.

Claims (12)

A laser light source capable of outputting a light wave of a specific wavelength;
The output light from the laser light source is input, and an external wavelength filter whose output light intensity changes according to the input wavelength,
The external wavelength filter has a configuration in which first and second wavelength filters having different wavelength periods of power transmittance are connected in cascade,
The first and second wavelength filters comprise first and second optical resonators having different wavelength resonance periods,
The first optical resonator includes a first air gap mirror formed in an optical waveguide optically coupled to the laser light source, and an optical waveguide on the output side of the first air gap mirror. A second air gap mirror formed at a location, and an optical waveguide between the first and second air gap mirrors,
The second optical resonator includes the second air gap mirror, a third air gap mirror formed at a location on the optical waveguide on the output side of the second air gap mirror, 2 and an optical waveguide between the third air gap mirrors,
The transmittance of the second air gap mirror is set lower than the transmittance of the first and third air gap mirrors, and the lengths of the first and second optical resonators are changed to change the first and second optical gap mirrors. 1, the wavelength resonance period of the second optical resonator is set to be different,
A light source for optical communication, wherein the light wave frequency-modulated in the laser light source is converted into intensity modulation by the external wavelength filter. The light source for optical communication according to claim 1 ,
The optical light source for optical communication, wherein the laser light source and the external wavelength filter are optically coupled via an optical isolator. The light source for optical communication according to claim 1 or 2 ,
The light source for optical communication, wherein the first and second optical resonators are optical waveguides formed on an SOI substrate. The light source for optical communication according to claim 1 or 2 ,
The first and second optical resonators comprise optical waveguides formed on a compound semiconductor substrate. The light source for optical communication according to any one of claims 1 to 4 ,
A heater disposed in the vicinity of the optical waveguide constituting the first and second optical resonators;
A power source capable of adjusting the power supplied to the heater;
A light source for optical communication, wherein the optical waveguide is heated by the heater to adjust the resonance peak wavelengths of the first and second optical resonators. The light source for optical communication according to any one of claims 1 to 4 ,
Furthermore, an electrode disposed in the vicinity of the optical waveguide constituting the first and second optical resonators,
A power source capable of adjusting the power supplied to the electrode;
A light source for optical communication, wherein the resonance peak wavelengths of the first and second optical resonators are adjusted by an electro-optic effect. The light source for optical communication according to claim 3 ,
The laser light source is a semiconductor laser;
The light source for optical communication, wherein the semiconductor laser and the external wavelength filter are hybrid-integrated on the same substrate. The light source for optical communication according to claim 4 ,
The laser light source is a semiconductor laser;
The light source for optical communication, wherein the semiconductor laser and the external wavelength filter are monolithically integrated on the same substrate. The light source for optical communication according to claim 8 ,
The active layer of the semiconductor laser and the waveguide core layer of the external wavelength filter are each composed of a multiple quantum well structure with different band gaps,
A band gap of an active layer of the semiconductor laser is set to be smaller than a band gap of a waveguide core layer of the external wavelength filter. The light source for optical communication according to claim 1 or 2 ,
The light source for optical communication, wherein the external wavelength filter is made of a quartz-based material. The light source for optical communication according to any one of claims 1 to 10 ,
The laser light source is a wavelength tunable light source capable of changing an oscillation wavelength. The light source for optical communication according to any one of claims 1 to 11 ,
And a laser driver for supplying an injection current modulated in accordance with an electrical signal to be transmitted to the laser light source,
A light source for optical communication, characterized in that a laser beam frequency-modulated from the laser light source is output.

Images