JP6771602B2 - Wavelength control element and wavelength control method - Google Patents

Description

The present invention relates to a wavelength control element and a wavelength control method suitable for use in the wavelength control element.

With the increase in the amount of information transmitted, optical wiring technology is drawing attention. In the optical wiring technology, an optical device using an optical fiber or an optical waveguide as a transmission medium is used to transmit information between devices, boards, chips, etc. in an information processing device by an optical signal. As a result, it is possible to improve the band limitation of electrical wiring, which is a bottleneck in information processing equipment that requires high-speed signal processing.

An optical device is configured to include optical elements such as an optical transmitter and an optical receiver. These optical elements can be spatially coupled to each other using, for example, a lens after performing complicated optical axis alignment for aligning the center position (light receiving position or light emitting position) of each optical element with the design position.

As a means for coupling each optical element, there is a technique of using an optical waveguide element instead of a lens. When an optical waveguide element is used, light is confined and propagated in the optical waveguide, so that unlike the case where a lens is used, complicated optical axis alignment is not required. Therefore, since the assembly process of the optical device is simplified, it is advantageous as a form suitable for mass production.

Here, for the optical waveguide element, for example, silicon (Si) can be used as the waveguide material. In an optical waveguide element (Si waveguide) made of Si, an optical waveguide core that is substantially a light transmission path is formed of Si as a material. Then, a clad made of, for example, silicon oxide (SiO ₂ ) having a refractive index lower than that of Si covers the periphery of the optical waveguide core. With such a configuration, the difference in refractive index between the optical waveguide core and the clad becomes extremely large, so that light can be strongly confined in the optical waveguide core. As a result, it is possible to realize a small curved waveguide in which the bending radius is reduced to, for example, about several μm. Therefore, it is possible to create an optical circuit having the same size as an electronic circuit, which is advantageous for miniaturization of the entire optical device.

Moreover, when using a Si waveguide, by diverting the manufacturing process of CMOS (Complementary Metal Oxide Semiconductor), it is possible to mass-produce optical devices in which elements having various functions are monolithically integrated on the same substrate. Is possible. Therefore, an optical device using a Si waveguide is advantageous in miniaturization and cost reduction (see, for example, Patent Document 1 and Non-Patent Document 1 and Non-Patent Document 2).

Generally, an optical module is used by being incorporated in an information processing device or the like. Therefore, when a semiconductor laser (LD) is integrated in an optical module, the LD is exposed to a harsh temperature environment and is affected by temperature changes due to heat generation due to its own power consumption. As a result, the oscillation wavelength of the LD fluctuates.

In wavelength division multiplexing (WDM) transmission in which an absolute wavelength is defined, it is necessary to take measures as an optical module so that the oscillation wavelength of the LD does not fluctuate. Therefore, a wavelength control element is incorporated in the optical module in which the LD is integrated (see, for example, Non-Patent Document 3).

The system described in Non-Patent Document 3 comprises two monitor light receiving elements (PD1 and PD2), a half mirror that divides light into two, and an etalon filter having periodic transmission characteristics with respect to wavelength. Will be done. This system uses the back light of the LD, divides the back light into two by a half mirror, and uses one light receiving element (PD1) for one light and an etalon filter with wavelength dependence for the other light. After passing through, it is detected by the other light receiving element (PD2). The received current value of PD2 has wavelength dependence because the incident light passes through the etalon filter. Therefore, the oscillation wavelength is stabilized by applying temperature control feedback to the LD so that the received current value of PD2 becomes constant. In such a wavelength control mechanism, it is necessary to hybridize functional blocks such as a half mirror and an etalon filter, and there is room for improvement in terms of mounting and element cost.

On the other hand, in Patent Document 2, a similar function is realized by using a quartz-based waveguide. The wavelength filter unit, which is one of the elemental devices of this system, is required to have temperature-independent characteristics. For this reason, a silicone resin or the like having a temperature dependence opposite to that of the waveguide is inserted in the wavelength filter portion described in Patent Document 2 in order to make it temperature independent. Further, the back light of the LD is used as the monitor light for wavelength control. Therefore, in this method, in addition to the alignment with respect to the front light which is the main output, the alignment with respect to the back light for the monitor is also required, which complicates the waveguide forming and mounting steps.

JP-A-2017-77133 Japanese Unexamined Patent Publication No. 2004-179465

IEEE Journal of selected topics in quantum electronics, vol.11, No.1, January / February 2005 p.232-240 IEEE Journal of selected topics in quantum electronics, vol.12, No.6, November / December 2006 p.1371-1379 Laser module with wavelength locker for WDM system (http://www.fujitsu.com/downloads/JP/archive/imgjp/jmag/vol51-3/paper04.pdf.)

As described above, the mounting process of each of the techniques disclosed in Non-Patent Document 3 or Patent Document 2 is complicated.

The present invention has been made in view of the above-mentioned problems. An object of the present invention is to provide a function for correcting an oscillation wavelength of a light source such as a semiconductor laser adjusted by initial setting when it deviates from the absolute wavelength due to some external factor, without complicating the process. It is to be composed of.

In order to achieve the above-mentioned object, the wavelength control element of the present invention includes a support substrate, an optical waveguide core, and a clad formed on the support substrate including the optical waveguide core.

The optical waveguide core has an optical dividing unit that divides the input light input from a light source having a wavelength control function into signal light and verification light, and the verification light is divided into two parts, a first verification light and a second verification light. It is provided with an optical branching portion for light generation and a wavelength filter portion for transmitting only a predetermined wavelength band of the second verification light. Further, the wavelength control element is a first light receiving element in which the first verification light is input to generate the first light receiving data, and a second light receiving element in which the second verification light transmitted through the wavelength filter unit is input to generate the second light receiving data. 2 The light receiving element, the difference acquisition unit that acquires the difference between the first light receiving data and the second light receiving data, and the difference are compared with the initial values stored in the storage unit in advance, and the light source is based on the comparison result. It is configured to include a comparison unit that sends a control signal to the wavelength control function of. Here, the optical dividing unit, the optical branching unit, and the wavelength filter unit are temperature-independent.

In a preferred embodiment of the wavelength control element of the present invention, the optical dividing section is provided on one side of the MMI waveguide and the MMI waveguide in the light propagation direction, and the input port into which the input light is input and the MMI. A first output port provided on the other side of the waveguide in the light propagation direction to output signal light, and an MMI waveguide provided on the other side of the light propagation direction to output verification light. It is equipped with a second output port. Further, the optical dividing portion is provided on one side of the tapered waveguide and the tapered waveguide in which the width gradually increases from one side in the light propagation direction to the other side, and is input. An input port for inputting light, a first output port provided on the other side of the tapered waveguide in the light propagation direction and outputting signal light, and the other side of the tapered waveguide in the light propagation direction. It may be configured to include a second output port provided on the side and from which verification light is output.

Further, in a preferred embodiment of the wavelength control element of the present invention, the optical branching portion is provided on one side of the MMI waveguide and the MMI waveguide in the light propagation direction, and has an input port into which verification light is input. , The first output port of the MMI waveguide, which is provided on the other side of the light propagation direction and outputs the first verification light, and the MMI waveguide, which is provided on the other side of the light propagation direction. It is configured to include a second output port from which two verification lights are output. Further, the optical branching portion is provided on one side of the tapered waveguide and the tapered waveguide in which the width gradually increases from one side in the light propagation direction to the other side, and is verified. An input port for inputting light and a first output port of the tapered waveguide provided on the other side of the light propagation direction to output the first verification light, and a tapered waveguide for the light propagation direction. A configuration may be provided in which a second output port is provided on the other side and a second verification light is output.

Further, in a preferred embodiment of the wavelength control element of the present invention, the wavelength filter unit connects a temperature-independent first optical coupler and a second optical coupler, and a first optical coupler and a second optical coupler, respectively. It is configured to include a first arm waveguide and a second arm waveguide.

From the first optical coupler side, the first arm waveguide includes a first curved waveguide, a first tapered waveguide, a second tapered waveguide, a first phase adjustment region, a third tapered waveguide, and a second curved waveguide. The third tapered waveguide, the first phase adjustment region, the second tapered waveguide, the first tapered waveguide, and the first curved waveguide are connected in this order, and the second arm waveguide is from the first optical coupler side. , 1st curved waveguide, 1st tapered waveguide, 2nd phase adjustment region, 2nd tapered waveguide, 3rd tapered waveguide, 2nd curved waveguide, 3rd tapered waveguide, 2nd tapered waveguide, 1st The two phase adjustment regions, the first tapered waveguide, and the first curved waveguide are connected in this order.

Here, the widths of the optical waveguide cores are different from each other in the first phase adjustment region and the second phase adjustment region, and the core width at one end of the first tapered waveguide is set to the core width of the first curved waveguide. The core width of the other end is equal and the core width of the other end is equal to the core width of the second phase adjustment region, and the core width of one end of the second tapered waveguide is equal to the core width of the second phase adjustment region and the other end. The core width of the third tapered waveguide is equal to the core width of the first phase adjustment region, and the core width of one end is equal to the core width of the first phase adjustment region and the core width of the other end is equal to the core width of the first phase adjustment region. , A tapered shape equal to the core width of the second curved waveguide, with an equivalent refractive index n _{1 of} light propagating in the first phase adjustment region and a length L ₁ along the propagation direction, and propagating in the second phase adjustment region. The equivalent refractive index n ₂ of the light to be generated and the length L ₂ along the propagation direction satisfy the following equations. In addition, T represents the temperature of the waveguide.

The first optical coupler included in the wavelength filter unit includes an MMI waveguide and a first input port provided on one side of the MMI waveguide in the light propagation direction and connected to a second output port of the optical branch. , A first output port of the MMI waveguide provided on the other side of the light propagation direction and connected to the first arm waveguide, and an MMI waveguide provided on the other side of the light propagation direction. The second optical coupler, which is configured to include a second output port connected to the second arm waveguide and is included in the wavelength filter unit, is located on one side of the MMI waveguide and the MMI waveguide in the light propagation direction. A first input port provided and connected to the first arm waveguide, and a second input port provided on one side of the MMI waveguide in the light propagation direction and connected to the second arm waveguide. It is preferable to have a configuration in which a first output port provided on the other side of the MMI waveguide in the light propagation direction and connected to the second light receiving element is provided. Further, the first optical coupler is provided on one side of the tapered waveguide and the tapered waveguide in which the width gradually increases from one side in the light propagation direction to the other side in the light propagation direction. An input port connected to the second output port of the optical branch, a first output port provided on the other side of the tapered waveguide in the light propagation direction and connected to the first arm waveguide, and a tapered guide. A configuration is provided in which the waveguide is provided on the other side of the light propagation direction and includes a second output port connected to the second arm waveguide, and the second optical coupler is mounted from one side of the light propagation direction to the other. A tapered waveguide whose width gradually narrows toward the side of the light, a first input port provided on one side of the tapered waveguide in the light propagation direction and connected to the first arm waveguide, and a tapered waveguide. A second input port provided on one side of the light propagation direction and connected to the second arm waveguide, and a second light receiving element provided on the other side of the tapered waveguide in the light propagation direction. It may be configured to include an output port connected to.

Further, the wavelength control method of the present invention of the present invention is a wavelength control method performed by using the above-mentioned wavelength control element, and is configured to include the following processes.

First, the signal light output from the first output unit of the optical division unit is monitored, and the wavelength of the light source is set to a desired wavelength. Next, the difference between the first light receiving data and the second light receiving data is stored in the storage unit as an initial value.

After that, when the difference between the newly acquired first light receiving data and the second light receiving data changes from the initial value, the wavelength of the light source is controlled so that the difference matches the initial value.

According to the wavelength control element of the present invention, when the oscillation wavelength of the semiconductor laser adjusted by the initial setting deviates from the absolute wavelength due to some external factor, the function for correcting it is provided by light without complicating the process. It can be configured with a circuit.

It is a schematic plan view which shows the wavelength control element. It is a schematic end view of a wavelength control element. It is a schematic diagram of an optical division part and an optical branch part. It is a schematic diagram (1) of the wavelength filter part. It is a schematic diagram (2) of the wavelength filter part. It is a schematic diagram for demonstrating the wavelength control function. It is a figure which shows the characteristic of an optical division part. It is a figure which shows the characteristic of an optical branch part. It is a figure which shows the characteristic of the wavelength filter part.

Hereinafter, embodiments of the present invention will be described with reference to the drawings, but the shape, size, and arrangement of each component are only schematically shown to the extent that the present invention can be understood. Further, although a preferable configuration example of the present invention will be described below, the material and numerical conditions of each component are merely suitable examples. Therefore, the present invention is not limited to the following embodiments, and many modifications or modifications can be made that can achieve the effects of the present invention without departing from the scope of the constitution of the present invention.

(Wavelength control element)
An embodiment of the wavelength control element of the present invention will be described with reference to FIGS. 1 and 2. FIG. 1 is a schematic plan view showing a wavelength control element. In FIG. 1, the support substrate and the cladding, which will be described later, are omitted. FIG. 2 is a schematic end view of the wavelength control element shown in FIG. 1 cut out along the line I-I.

The wavelength control element 100 has an optical waveguide including a support substrate 10, a clad 20, and an optical waveguide core 30 as a basic structure.

The support substrate 10 is made of, for example, a flat plate made of single crystal Si.

The clad 20 is provided on the support substrate 10. The clad 20 covers the upper surface of the support substrate 10 and is formed including the optical waveguide core 30. The clad 20 is formed of, for example, silicon oxide (SiO ₂ ) as a material.

The optical waveguide core 30 is formed of, for example, Si, which has a higher refractive index than the clad 20. As a result, the optical waveguide core 30 functions as a light transmission path, and the light input to the optical waveguide core 30 propagates in the propagation direction according to the planar shape of the optical waveguide core 30.

The optical waveguide core 30 includes an optical dividing unit 200, an optical branching unit 300, and a wavelength filter unit 400. Further, the wavelength control element 100 includes a first light receiving element (PD1) 501, a second light receiving element (PD2) 502, and a wavelength control unit 600. The wavelength control unit 600 further includes a difference acquisition unit 602, a comparison unit 604, and a storage unit 606. Each component of the wavelength control unit 600 can be optionally configured by those skilled in the art.

The optical dividing unit 200 has an MMI waveguide 201, an input port 202, a first output port 203, and a second output port 204. The input port 202 is provided on one side of the light propagation direction of the MMI waveguide 201. Further, the first output port 203 and the second output port 204 are provided on the other side of the MMI waveguide 201 in the light propagation direction.

For example, a semiconductor laser (LD) 110 is connected to the input port 202 of the optical division unit 200 as a transmission light source. The optical division unit 200 divides the input light (laser output light) input through the input port 202 into signal light and verification light. The signal light is output from the first output port 203, and the verification light is output from the second output port 204.

When a part of the laser output light is used for wavelength control, the light dividing unit 200 has a structure in which the loss of light is suppressed as much as possible, the characteristics do not fluctuate with respect to wavelength and temperature, and the manufacturing error is strong. It is preferable to have. Therefore, it is preferable that the optical dividing unit 200 has a 1 × 2 MMI structure with 1 input and 2 outputs.

The length of the MMI waveguide 201 is determined by the beat lengths of the basic mode and the primary mode propagating the MMI waveguide 201. Further, by setting the offset amount in the width direction of the input port 202 with respect to the MMI waveguide 201 to the same amount as that of the first output port 203, a mirror image of the light input to the input port 202 can be obtained at the first output port 203. .. In this way, the first output port 203 functions as a low-loss transmission port, and outputs the input laser output light as signal light with low loss. The second output port 204 outputs the remaining scattered components as verification light.

The signal light output from the first output port 203 is output to the outside of the wavelength control element. This signal light is input to, for example, a modulator for signal generation and becomes an output signal light. A modulator for signal generation may be mounted on the wavelength control element 100. Further, the verification light output from the second output port 204 is sent to the optical branching unit 300.

As described above, the intensity a of the signal light and the intensity b (= 1-a) of the verification light output from the optical dividing unit 200 are designed to satisfy the relationship of a> b. If the intensity b of the verification light is large, the signal is lost by that amount, and the intensity a of the signal light becomes small. Therefore, the intensity b of the verification light should be as small as possible within a range sufficient for wavelength control.

The optical branching unit 300 has an MMI waveguide 301, an input port 302, a first output port 303, and a second output port 304. The input port 302 is provided on one side of the light propagation direction of the MMI waveguide 301. Further, the first output port 303 and the second output port 304 are provided on the other side of the MMI waveguide 301 in the light propagation direction.

The second output port 204 of the optical dividing unit 200 is connected to the input port 302 of the optical branching unit 300. The optical branching unit 300 equally branches the verification light input via the input port 302 into the first verification light and the second verification light. The first verification light is output from the first output port 303, and the second verification light is output from the second output port 304.

Since the optical branching portion 300 evenly branches the verification light into the first verification light and the second verification light, the input port 302 is arranged at the center of the MMI waveguide 301 in the width direction. Further, the first output port 303 and the second output port 304 are arranged so as to be equidistant from the center in the width direction of the MMI waveguide 301.

Like the optical dividing unit 200, the optical branching unit 300 is also required to have flat characteristics with respect to wavelength and temperature and to be resistant to manufacturing errors. Therefore, it is preferable that the optical branching portion 300 has a 1 × 2 MMI structure with 1 input and 2 outputs.

The length of the MMI waveguide 301 is determined so that the output ratios of the first output port 303 and the second output port 304 are equally branched.

The first verification light output from the first output port 303 is sent to the first light receiving element (PD1). Further, the second verification light output from the second output port 304 is sent to the wavelength filter unit 400.

Here, an example in which the optical division section 200 and the optical branch section 300 have a 1 × 2 MMI structure has been described, but a Y-branch waveguide structure may be used.

The optical division portion and the optical branch portion when the Y-branch waveguide structure is adopted will be described with reference to FIG. FIG. 3A is a schematic view of the optical branching portion, and FIG. 3B is a schematic diagram of the optical branching portion. As shown in FIG. 3A, the optical dividing unit 210 includes a tapered waveguide 211, an input port 212, a first output port 213, and a second output port 214. The tapered waveguide 211 has a tapered structure in which the width gradually increases from one side in the light propagation direction toward the other side in the light propagation direction. An input port 212 is provided on one side of the tapered waveguide 211 in the light propagation direction. Further, a first output port 213 and a second output port 214 are provided on the other side of the tapered waveguide 211 in the light propagation direction.

The size of the surface of the first output port 213 and the second output port 214 connected to the tapered waveguide 211 is larger in the first output port 213 than in the second output port 214. With this configuration, the intensity a of the signal light and the intensity b (= 1-a) of the verification light output from the optical dividing unit 210 are a> b.

As shown in FIG. 3B, the optical branching portion 310 includes a tapered waveguide 311, an input port 312, a first output port 313, and a second output port 314. The tapered waveguide 311 has a tapered structure in which the width gradually increases from one side in the light propagation direction toward the other side in the light propagation direction. An input port 312 is provided on one side of the tapered waveguide 311 in the light propagation direction. Further, a first output port 313 and a second output port 314 are provided on the other side of the tapered waveguide 311 in the light propagation direction.

The sizes of the surfaces of the first output port 313 and the second output port 314 connected to the tapered waveguide 311 are equal to each other. With this configuration, the intensities of the first detection light and the second detection light output from the optical branch portion 310 are equal to each other, and the optical branch portion 310 functions as a 3 dB coupler.

When the optical dividing portion and the optical branching portion have a 1 × 2 MMI structure, the light loss in the optical dividing portion and the optical branching portion can be reduced because the light is divided or branched by utilizing the interference of light. On the other hand, when the optical dividing portion and the optical branching portion have a Y-branched structure, optical loss occurs in the gap between the first output port and the second output port, but it becomes easy to suppress the loss due to the manufacturing error.

The wavelength filter unit will be described with reference to FIG. FIG. 4 is a schematic plan view of the wavelength filter unit.

The wavelength filter unit 400 includes a first optical coupler 410, a second optical coupler 420, a first arm waveguide 430, and a second arm waveguide 440. The first arm waveguide 430 and the second arm waveguide 440 connect the first optical coupler 410 and the second optical coupler 420, respectively. The wavelength filter unit 400 has a Mach-Zehnder interferometer (MZI) structure.

The first optical coupler 410 includes an MMI waveguide 411, a first input port 412, a second input port 413, a first output port 414, and a second output port 415. A first input port 412 and a second input port 413 are provided on one side of the MMI waveguide 411 in the optical propagation direction, and a first output port 414 and a second output port 415 are provided on the other side in the optical propagation direction. It is provided. The first optical coupler 410 is composed of, for example, a 2 × 2 MMI structure that outputs the light input to the first input port 412 from the first output port 414 and the second output port 415 at an equal branch ratio.

Similarly, the second optical coupler 420 includes an MMI waveguide 421, a first input port 422, a second input port 423, a first output port 424, and a second output port 425. A first input port 422 and a second input port 423 are provided on one side of the MMI waveguide 421 in the optical propagation direction, and a first output port 424 and a second output port 425 are provided on the other side in the optical propagation direction. It is provided. The second optical coupler 420 has a 2 × 2 MMI structure that outputs the light input to the first input port 422 and the second input port 423 from the first output port 424 and the second output port 425 at equal branch ratios, respectively. Consists of.

For example, the first input port 412 of the first optical coupler 410 is used as an input port of the wavelength filter unit 400. Further, the first output port 424 of the second optical coupler 420 is used as an output port of the wavelength filter unit 400.

Here, by configuring the first optical coupler 410 and the second optical coupler 420 with a 2 × 2 MMI structure, the temperature characteristics of the first optical coupler 410 and the second optical coupler 420 become flat.

From the first optical coupler 410 side, the first arm waveguide 430 includes a first curved waveguide 431, a first tapered waveguide 432, a second tapered waveguide 433, a first phase adjustment region 434, and a third tapered waveguide 435. , 2nd curved waveguide 436, 3rd tapered waveguide 435, 1st phase adjustment region 434, 2nd tapered waveguide 433, 1st tapered waveguide 432 and 1st curved waveguide 431 are connected in this order. .. Further, from the first coupler 410 side, the second arm waveguide section 440 includes a first curved waveguide 431, a first tapered waveguide 432, a second phase adjustment region 444, a second tapered waveguide 433, and a third taper guide. The waveguide 435, the second curved waveguide 436, the third tapered waveguide 435, the second tapered waveguide 433, the second phase adjustment region 444, the first tapered waveguide 432, and the first curved waveguide 431 are connected in this order. Will be done.

The portion of the first arm waveguide section 430 other than the first phase adjustment region 434, that is, the first curved waveguide 431, the first tapered waveguide 432, the second tapered waveguide 433, the third tapered waveguide 435, and the like. The second curved waveguide 436 is also referred to as a first routing waveguide. Further, in the second arm waveguide section 440, the portion other than the second phase adjustment region 444, that is, the first curved waveguide 431, the first tapered waveguide 432, the second tapered waveguide 433, and the third tapered waveguide. The 435 and the second curved waveguide 436 are also referred to as a second routing waveguide.

The width (core width) of the optical waveguide core is different between the first phase adjustment region 434 and the second phase adjustment region 444. The core width of one end of the first tapered waveguide 432 is equal to the core width of the first curved waveguide 431, and the core width of the other end is equal to the core width of the second phase adjustment region 444. The core width of one end of the second tapered waveguide 433 is equal to the core width of the second phase adjustment region 444, and the core width of the other end is equal to the core width of the first phase adjustment region 434. Further, in the third tapered waveguide 435, the core width at one end is equal to the core width of the first phase adjustment region 434, and the core width at the other end is equal to the core width of the second curved waveguide 436.

The first routing waveguide and the second routing waveguide have the same structure and the same length. Therefore, in the first routing waveguide and the second routing waveguide, there is no phase difference between the propagating light. That is, the first arm waveguide section 430 is composed of a first phase adjustment region 434 and a first routing waveguide that does not contribute to phase adjustment. Further, the second arm waveguide section 440 is composed of a second phase adjustment region 444 and a second routing waveguide that does not contribute to phase adjustment. Therefore, the wavelength selection function of the wavelength filter unit 400 is provided by phase interference between the light propagating in the first arm waveguide 430 and the light propagating in the second arm waveguide 440.

Since the core widths of the first arm waveguide 430 and the second arm waveguide 440 change continuously due to the first to third tapered waveguides 432, 433, and 435, light loss can be suppressed. ..

In order to make the wavelength selection characteristic of the wavelength filter unit 400 flat with respect to the temperature change, that is, temperature-independent, the first phase adjustment region 434 in the first arm waveguide 430 and the second in the second arm waveguide 440 The relative relationship of the length in the light propagation direction with the phase adjustment region 444 is such that the phase difference of the light given by the first phase adjustment region 434 and the second phase adjustment region 444 does not fluctuate with respect to the temperature change of the waveguide. Is set to. The temperature dependence of the optical phase difference between the two is given by the following equation (1).

Here, φ, λ, and T represent the phase difference given in the first phase adjustment region 434 and the second phase adjustment region 444, the wavelength of light, and the temperature of the waveguide, respectively. n ₁ and L ₁ represent the equivalent refractive index of the light propagating in the first phase adjustment region 434 and the length along the propagating direction, respectively. Further, n ₂ and L ₂ represent the equivalent refractive index of the light propagating in the second phase adjustment region 444 and the length along the propagating direction, respectively.

In the above equation (1), when a condition is imposed so that the slope of the phase with respect to the temperature, that is, the right side becomes 0, it is a relative relational expression of the lengths of the first phase adjustment region 434 and the second phase adjustment region 444. The following equation (2) is obtained.

In order to give different structures in the first phase adjustment region 434 and the second phase adjustment region 444, in this configuration example, the optical waveguide core widths of the first phase adjustment region 434 and the second phase adjustment region 444 are different from each other. It is set to a value. Then, the relative relationship between the lengths of the first phase adjustment region 434 and the second phase adjustment region 444 may be determined so as to satisfy the above equation (2).

Here, an example in which the first optical coupler 410 and the second optical coupler 420 have a 2 × 2 MMI structure has been described, but a Y-branched waveguide structure may be used.

With reference to FIG. 5, the wavelength filter unit of another configuration example when the Y-branch waveguide structure is adopted will be described. FIG. 5 is a schematic view of a wavelength filter unit of another configuration example.

The wavelength filter unit shown in FIG. 5 differs from the wavelength filter unit described with reference to FIG. 4 in the configurations of the first optical coupler and the second optical coupler. Since the other parts can be configured in the same manner, the description thereof will be omitted.

As shown in FIG. 5, the first optical coupler 460 includes a tapered waveguide 461, an input port 462, a first output port 463, and a second output port 464. The tapered waveguide 461 has a tapered structure in which the width gradually increases from one side in the light propagation direction toward the other side in the light propagation direction. An input port 462 is provided on one side of the tapered waveguide 461 in the light propagation direction. Further, a first output port 463 and a second output port 463 are provided on the other side of the tapered waveguide 461 in the light propagation direction.

The first output port 463 is connected to the first arm waveguide 430. Further, the second output port 464 is connected to the second arm waveguide 440.

Further, the second optical coupler 470 includes a tapered waveguide 471, a first input port 472, a second input port 473, and an output port 474. The tapered waveguide 471 has a tapered structure in which the width gradually narrows from one side in the light propagation direction toward the other side in the light propagation direction. A first input port 472 and a second input port 473 are provided on one side of the tapered waveguide 471 in the light propagation direction. Further, an output port 474 is provided on the other side of the tapered waveguide 471 in the light propagation direction.

The first input port 472 is connected to the first arm waveguide 430. Further, the second input port 473 is connected to the second arm waveguide 440.

The input port 462 of the first optical coupler 460 serves as the input port of the wavelength filter unit 400, and the output port 474 of the second optical coupler 470 serves as the output port of the wavelength filter unit 400.

When the first optical coupler and the second optical coupler have a 2 × 2 MMI structure, the extinction ratio can be increased because light interference is used. Further, the light for the monitor can be taken out from the second output port of the second optical coupler. On the other hand, when the first optical coupler and the second optical coupler have a Y-branched structure, it becomes easy to suppress the loss due to the manufacturing error.

The input port of the wavelength filter unit 400 is connected to the second output port 304 of the optical branch unit 300. The second verification light output from the optical branching unit 300 is input to the input port of the wavelength filter unit 400.

The light output from the output port of the wavelength filter unit 400 is sent to the second light receiving element (PD2) 502.

As the first light receiving element (PD1) 501 and the second light receiving element (PD2) 502, a waveguide type Ge photodiode obtained by selectively growing germanium (Ge) on the Si optical waveguide core can be used. As the output of the first light receiving element (PD1) 501 and the second light receiving element (PD2) 502, for example, the current value corresponding to the input light intensity is the wavelength control unit as the first light receiving data and the second light receiving data, respectively. Sent to 600. The difference acquisition unit 602 provided in the wavelength control unit 600 converts the current value into electric power [dBm] from the conversion efficiency [A / W] of each light receiving element. After that, the difference between the first light receiving data and the second light receiving data converted into electric power is sent to the comparison unit 604.

The comparison unit 604 reads the initial value of the difference between the first light receiving data and the second light receiving data from the storage unit 606 and compares it with the difference sent from the difference acquisition unit 602.

The wavelength control function will be described with reference to FIG. FIG. 6 is a schematic diagram for explaining the function of wavelength control. In FIG. 6, the horizontal axis represents the wavelength [arbitrary unit], and the vertical axis represents the electric power [unit: dBm].

When there is a change in the oscillation wavelength of the LD110 due to an external factor such as temperature fluctuation, the first light receiving data (indicated by PD1 in the figure) shows a substantially constant value. This is because the first verification light input to the first light receiving element 501 is input to the first light receiving element 501 via the optical dividing unit 200 and the optical branching unit 300, which have flat characteristics in temperature and wavelength. Is. On the other hand, the second light receiving data (indicated by PD2 in the figure) changes according to the wavelength. This is because the second verification light input to the second light receiving element 502 is flat with respect to temperature after passing through the optical splitting section 200 and the optical branching section 300, which have flat characteristics in temperature and wavelength. This is because the light is input to the second light receiving element 502 via the wavelength filter 400 having wavelength selectivity.

Therefore, when there is a change in the oscillation wavelength of the LD 110, the difference sent from the difference acquisition unit 602 deviates from the initial value. Since the transmission characteristic of the wavelength filter unit 400 does not change with respect to temperature fluctuations, the change in the difference is specified to be purely due to the shift of the oscillation wavelength of the LD 110.

Therefore, the comparison unit 604 changes the temperature of the LD 110 by changing the voltage applied to the Peltier element mounted on the LD 110 so that the difference sent from the difference acquisition unit 602 matches the initial value. Controls the wavelength of the LD110.

(Wavelength control method)
A wavelength control method using the above-mentioned wavelength control element will be described.

First, the signal light output from the first output unit 203 of the optical division unit 200 is monitored by using a spectrum analyzer or the like, and the temperature of the Peltier element mounted on the LD 110 is adjusted to output the light from the LD 110. The wavelength of the laser output light is set to a desired wavelength.

Next, the difference acquisition unit 602 receives the first light receiving data and the second light receiving data from the first light receiving element (PD1) and the second light receiving element (PD2). The difference acquisition unit 602 acquires the difference between the first light receiving data and the second light receiving data converted by electric power, and stores it in the storage unit 606 as an initial value.

After that, the difference acquisition unit 602 acquires the difference in power between the first light receiving data and the second light receiving data, which has been converted into electric power, and the comparison unit 604 compares it with the initial value stored in the storage unit 606.

When the acquired difference changes from the initial value, the comparison unit 604 controls the wavelength of the LD 110 so that the difference matches the initial value.

According to the wavelength control element and the wavelength control method described above, the function of monitoring the wavelength of the laser output light of the LD mounted on the optical module and providing temperature control feedback to stabilize the wavelength is realized by the waveguide process. Can be done. In addition, the temperature-independent characteristics required for the wavelength filter unit can be achieved only by combining the width dimensions of the waveguide, and the manufacturing process is not complicated. Further, a part of the laser output light output from the front surface of the LD is used for wavelength control, and the back light is not used. Therefore, the alignment of the LD only needs to be performed on the front surface, and the mounting process can be simplified.

(Characteristic evaluation)
The characteristics of the optical division unit, the optical branching unit, and the wavelength filter unit that constitute the wavelength control element will be described.

A simulation performed by using the Finite Differential Time Domain (FDTD) method for the optical division will be described. The result of the simulation is shown in FIG. FIG. 7 is a diagram showing the transmittance of the light dividing portion, in which the horizontal axis represents the wavelength [unit: μm] and the vertical axis represents the transmittance [unit: dBm].

Here, the design wavelength band of the wavelength control element is set to around 1550 nm. Further, the thickness of the SOI layer of the Silicon on Insulator (SOI) substrate used for manufacturing the waveguide, that is, the thickness of the optical waveguide core of Si was set to 220 nm.

Further, the optical dividing unit 200 has a 1 × 2 MMI structure with 1 input and 2 outputs, the width of the MMI waveguide 201 is 1.5 μm, and the length along the light propagation direction is 4.098 μm.

As shown in FIG. 7, the output ratio of the first output port (portA) 203 and the second output port (portB) 204 is approximately 5 (-1 dB): 1 (-8 dB) with respect to the wavelength. It turns out that it is flat (that is, it is also flat in temperature).

Next, a simulation performed by using the FDTD method for the optical branch portion will be described. The result of the simulation is shown in FIG. FIG. 8 is a diagram showing the transmittance of the optical branch portion, in which the horizontal axis represents the wavelength [unit: μm] and the vertical axis represents the transmittance [unit: dBm].

The optical branch 300 has a 1 × 2 output MMI structure, the width of the MMI waveguide 301 is 1.5 μm, and the length along the light propagation direction is 2.049 μm.

As shown in FIG. 8, the output ratio of the first output port (portA) 303 and the second output port (portB) 304 is equally branched to about 1 (-3 dB): 1 (-3 dB), and the wavelength is It can be seen that the temperature is flat (that is, the temperature is also flat).

Next, a simulation performed by using the FDTD method for the wavelength filter unit will be described. The result of the simulation is shown in FIG. FIG. 9 is a diagram showing the output characteristics of the wavelength filter unit, in which the horizontal axis represents the wavelength [unit: μm] and the vertical axis represents the output intensity [unit: dBm]. In FIG. 9, the spectra obtained by oscillating the temperature by ± 50 K with respect to the reference temperature of 20 ° C. are superimposed.

By making the first arm waveguide 430 and the second arm waveguide 440 common components other than the first phase adjustment region 434 and the second phase adjustment region 444, the first arm waveguide 430 and the second arm guide are used. The optical phase difference with the waveguide 440 can be simplified as the phase difference between the first phase adjustment region 434 and the second phase adjustment region 444. In order to give different structures in the first phase adjustment region 434 and the second phase adjustment region 444, the simplest method of different width dimensions of the optical waveguide core was selected. For example, the cross-sectional width dimension of the waveguide in the first phase adjustment region 434 is set to 1 μm, and the cross-sectional width dimension of the waveguide in the second phase adjustment region 444 is set to 0.4 μm. With respect to this width, the temperature dependence of the mode-equivalent refractive index propagating in the first phase adjustment region 434 and the second phase adjustment region 444 related to the above equation (2) was calculated, and L ₂ = 0.9685 L ₁ was obtained. It was. For the spectral characteristics of the wavelength filter unit 400, the absolute value of L ₁ may be used as a parameter while maintaining the relative relationship between L ₁ and L ₂ obtained from the above equation (2). Here, L ₁ = 220 μm. ..

As shown in FIG. 9, the amount of fluctuation of the wavelength when the temperature fluctuates by ± 50 K is about ± 0.1 nm, and the temperature dependence is Δλ / ΔT = 0.002 nm / K, which is a spectral change with respect to the temperature fluctuation. Can be seen to be slowing down. Since the temperature dependence of a general Si waveguide without temperature countermeasures is Δλ / ΔT = 0.07 nm / K, the temperature dependence of the wavelength filter unit is suppressed to 1/35. I understand.

10 Support board
20 clad
30 Optical waveguide core
100 Wavelength control element 110 Semiconductor laser (LD)
200 Optical dividing unit 300 Optical branching unit 400 Wavelength filter unit 501 First light receiving element 502 Second light receiving element 600 Wavelength control unit 602 Difference acquisition unit 604 Comparison unit 606 Storage unit

Claims (8)

Support board and
Optical waveguide core and
A clad formed on the support substrate including the optical waveguide core is provided.
The optical waveguide core
An optical divider that divides the input light input from a light source equipped with a wavelength control function into signal light and verification light.
An optical branching portion that splits the verification light into a first verification light and a second verification light, and
A wavelength filter unit that transmits only a predetermined wavelength band of the second verification light is provided.
further,
The first light receiving element to which the first verification light is input to generate the first light receiving data, and
A second light receiving element that generates second light receiving data by inputting the second verification light that has passed through the wavelength filter unit, and
A difference acquisition unit that acquires a difference between the first light receiving data and the second light receiving data,
The difference is compared with an initial value stored in a storage unit in advance, and based on the result of the comparison, a comparison unit for sending a control signal to the wavelength control function of the light source is provided.
The light dividing unit, the light branching unit, and the wavelength filter unit, Ri athermal der,
The wavelength filter unit
Temperature-independent first and second optical couplers,
Each includes a first arm waveguide and a second arm waveguide that connect the first optical coupler and the second optical coupler.
From the first optical coupler side, the first arm waveguide includes a first curved waveguide, a first tapered waveguide, a second tapered waveguide, a first phase adjustment region, a third tapered waveguide, and a second curved guide. It is composed of a waveguide, a third tapered waveguide, a first phase adjustment region, a second tapered waveguide, a first tapered waveguide, and a first curved waveguide connected in this order.
From the first optical coupler side, the second arm waveguide includes a first curved waveguide, a first tapered waveguide, a second phase adjustment region, a second tapered waveguide, a third tapered waveguide, and a second curved guide. It is composed of a waveguide, a third tapered waveguide, a second tapered waveguide, a second phase adjustment region, a first tapered waveguide, and a first curved waveguide connected in this order.
The widths of the optical waveguide cores of the first phase adjustment region and the second phase adjustment region are different from each other.
The first tapered waveguide has a tapered shape in which the core width at one end is equal to the core width of the first curved waveguide and the core width at the other end is equal to the core width of the second phase adjustment region.
The second tapered waveguide has a tapered shape in which the core width at one end is equal to the core width of the second phase adjustment region and the core width at the other end is equal to the core width of the first phase adjustment region.
The third tapered waveguide has a tapered shape in which the core width at one end is equal to the core width of the first phase adjustment region and the core width at the other end is equal to the core width of the second curved waveguide.
The equivalent refractive index n ₁ of the light propagating in the first phase adjustment region and the length L ₁ along the propagation direction, and the equivalent refractive index n ₂ and the propagation direction of the light propagating in the second phase adjustment region . The length L ₂ is a wavelength control element characterized by satisfying the following equation .

The optical split section
MMI waveguide and
An input port provided on one side of the MMI waveguide in the light propagation direction and into which the input light is input,
A first output port provided on the other side of the MMI waveguide in the light propagation direction and from which the signal light is output,
The wavelength control element according to claim 1, further comprising a second output port provided on the other side of the MMI waveguide in the light propagation direction and from which the verification light is output. The optical split section
A tapered waveguide in which the width gradually increases from one side to the other in the light propagation direction,
An input port provided on one side of the tapered waveguide in the light propagation direction and into which the input light is input,
A first output port provided on the other side of the tapered waveguide in the light propagation direction and from which the signal light is output,
The wavelength control element according to claim 1, further comprising a second output port provided on the other side of the tapered waveguide in the light propagation direction and from which the verification light is output. The optical branch is
MMI waveguide and
An input port provided on one side of the MMI waveguide in the light propagation direction and into which the verification light is input,
A first output port provided on the other side of the MMI waveguide in the light propagation direction and from which the first verification light is output,
The invention according to any one of claims 1 to 3, further comprising a second output port provided on the other side of the MMI waveguide in the light propagation direction and from which the second verification light is output. The wavelength control element described. The optical branch is
A tapered waveguide in which the width gradually increases from one side to the other in the light propagation direction,
An input port provided on one side of the tapered waveguide in the light propagation direction and into which the verification light is input,
A first output port provided on the other side of the tapered waveguide in the light propagation direction and from which the first verification light is output,
The invention according to any one of claims 1 to 3, further comprising a second output port provided on the other side of the tapered waveguide in the light propagation direction and from which the second verification light is output. The wavelength control element described. The first optical coupler is
MMI waveguide and
A first input port provided on one side of the MMI waveguide in the light propagation direction and connected to a second output port of the optical branching portion, and a first input port.
A first output port provided on the other side of the MMI waveguide in the light propagation direction and connected to the first arm waveguide.
It is provided with a second output port provided on the other side of the MMI waveguide in the light propagation direction and connected to the second arm waveguide.
The second optical coupler
MMI waveguide and
A first input port provided on one side of the MMI waveguide in the light propagation direction and connected to the first arm waveguide.
A second input port provided on one side of the MMI waveguide in the light propagation direction and connected to the second arm waveguide.
The invention according to any one of claims 1 to 5 , further comprising a first output port provided on the other side of the MMI waveguide in the light propagation direction and connected to the second light receiving element. The wavelength control element described. The first optical coupler is
A tapered waveguide in which the width gradually increases from one side to the other in the light propagation direction,
An input port provided on one side of the tapered waveguide in the light propagation direction and connected to a second output port of the optical branching portion, and an input port.
A first output port provided on the other side of the tapered waveguide in the light propagation direction and connected to the first arm waveguide.
It is provided with a second output port provided on the other side of the tapered waveguide in the light propagation direction and connected to the second arm waveguide.
The second optical coupler
A tapered waveguide whose width gradually narrows from one side to the other in the light propagation direction,
A first input port provided on one side of the tapered waveguide in the light propagation direction and connected to the first arm waveguide.
A second input port provided on one side of the tapered waveguide in the light propagation direction and connected to the second arm waveguide.
The invention according to any one of claims 1 to 5, wherein the tapered waveguide is provided on the other side in the light propagation direction and includes an output port connected to the second light receiving element. Wavelength control element. A wavelength control method performed by using the wavelength control element according to any one of claims 1 to 7 .
The method comprising monitoring the light splitting unit or al the signal light output, and sets the wavelength of the light source to a desired wavelength,
A process of storing the difference between the first light receiving data and the second light receiving data as an initial value in the storage unit, and
When the difference between the newly acquired first light receiving data and the second light receiving data changes from the initial value, the wavelength of the light source is controlled so that the difference matches the initial value. A wavelength control method characterized by including a process.

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