JP2006053436A - Wavelength variable filter, characteristic controlling method of wavelength variable filter and wavelength variable laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wavelength variable filter wherein a narrow-band wavelength and low loss can be realized and to provide a wavelength variable laser using the wavelength variable filter. <P>SOLUTION: The wavelength variable filter is provided with a pair of an input waveguide and an output waveguide, optical couplers disposed at a fixed interval on both the input waveguide and the output waveguide and a ladder interference type filter consisting of N (N;a natural number of 3 or more) pieces of array waveguides connecting the input waveguide to the output waveguide via the optical couplers. One or a plurality of low diffraction order regions having a fixed difference of optical path length successively from the array waveguide of a first step connected to an incident end side of the output waveguide and a high diffraction order region having fixed differences of only i pieces of array waveguides continued from an M step (M;a natural number of N-1 or less, i;a natural number of 1 or more and M+i; a natural number of N or less) so that the difference of the optical path length of the array waveguide of (M+i)th step is p-th (p;a natural number) multiple of that of the array waveguide of M-th step are connected in series. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a tunable filter, a filter characteristic control method, and a semiconductor tunable laser, which are important optical components of a wavelength division multiplexing optical communication system.

  In recent years, with the explosive increase of Internet traffic, a high-density wavelength division multiplexing optical transmission system has been introduced. In such a high-density wavelength division multiplexing transmission, a semiconductor laser used as a light source is strongly required to have a broadband wavelength variability from the viewpoint of cost performance, and a super-structure grating (SSG) has been used so far. Wavelength tunable laser used (for example, see Non-Patent Document 1), wavelength tunable laser (for example, see Non-Patent Document 2) using an arrayed waveguide grating (AWG), ladder interference filter, and ring resonator A wavelength tunable laser (see Non-Patent Document 3) using a laser is being researched and developed.

  FIG. 15 shows a structural diagram of a wavelength tunable laser using a conventional SSG. SSG is a grating having reflection characteristics at a constant wavelength interval (FSR: free-spectral range). In a wavelength tunable laser using SSG, by using SSG having two different FSRs, wavelength tunable operation is possible within a wavelength range corresponding to the least common multiple of each FSR. In the case of a wavelength tunable laser using a conventional SSG, it is necessary to control the refractive index of the phase adjustment region as well as the two SSG regions during the wavelength tunable operation.

  FIG. 16 shows a configuration of a wavelength tunable laser using a conventional AWG. In the case of a multi-wavelength selective light source using AWG, the oscillation mode becomes unstable because the longitudinal mode interval is narrowed to about 3 GHz due to an increase in semiconductor optical amplifier (SOA) with respect to the number of oscillation wavelengths and a relatively long resonator length. The problem was concerned.

  FIG. 17 shows a configuration of a wavelength tunable laser 1700 using a conventional ladder interference filter 1720 and a ring resonator 1760. A wavelength tunable laser composed of a combination of a ladder interference filter and a ring resonator has a feature that digitally varies the wavelength of a certain wavelength interval by a simple control method.

  However, in this wavelength tunable laser, it is necessary to sufficiently narrow the band of the ladder interference type wavelength filter in order to stabilize the oscillation wavelength. The most effective method for narrowing the band of the ladder interference type wavelength filter includes increasing the number of arrayed waveguides and increasing the diffraction order of the ladder interferometer.

  Table 1 shows the effects on the ladder interference filter characteristics when the number of array waveguides and the diffraction order are increased. Filter loss, laser longitudinal mode interval, ladder filter transmission wavelength interval (FSR), and wavelength variable range Shown from the perspective.

  When the number of arrayed waveguides is increased, the insertion loss increases as the element size and optical coupler increase, and the longitudinal mode interval of the laser becomes narrower due to the increase in the optical path length. The bandwidth required for the wavelength lock filter is further narrowed.

  Further, as the number of array waveguides increases, the number of optical couplers inevitably increases, but the optical coupler usually has a wavelength dependency in which the coupling coefficient changes depending on the optical wavelength. Therefore, as the number of optical couplers increases, the wavelength dependence of the ladder interference filter characteristics becomes more conspicuous. Therefore, there is a concern that the output power may not be kept constant along with the wavelength variation in the application of the wavelength tunable laser. Is done.

  On the other hand, in the case of narrowing the band by increasing the diffraction order, the loss and the longitudinal mode interval are kept constant, but there is a problem that the FSR and the wavelength variable range decrease in inverse proportion to the diffraction order. Accordingly, when the gain band of the SOA used in the wavelength tunable laser is wider than the FSR of the ladder interference filter, destabilization of laser oscillation characteristics such as mode hops becomes a problem. The problem of the narrow FSR can be solved by introducing a chirping technique (for example, see Non-Patent Document 2) that has been studied with a laser using AWG.

  However, since the wavelength variable range is inversely proportional to the diffraction order of the ladder interferometer, wideband wavelength variable operation cannot be performed as long as the diffraction order is increased. Eventually, it is difficult to realize a ladder interference type wavelength filter that enables oscillation wavelength stabilization, low loss (high power oscillation), and broadband wavelength tunable operation regardless of which method is used.

H.Ishii et al, “Multiple-Phase-Shift Super Structure Grating DBR Lasers for Broad Wavelength Tuning”, IEEE Photonics Technology Letters, Vol.5, No.6, June 1993, pp.613-615 C.R.Doerr et al, “Chirped Waveguide Granting Router Multifrequency Laser with Absolute Wavelength Control”, IEEE Photonics Technology Letters, Vol.8, No.12, December 1993, pp.1606-1608 S. Matsuo et al, “Digitally tunable laser using ladder filter and ring resonator”, Proceedings ECOC 2003, September 2003, pp884-885

  An object of the present invention is to provide a ladder interference type wavelength filter capable of narrowing the band, reducing the loss, and increasing the wavelength variable range. It is another object of the present invention to provide a wavelength tunable laser capable of stabilizing the oscillation wavelength, performing high output operation, and broadband wavelength tunable operation by using the ladder interference type wavelength filter as an oscillation wavelength selection element.

  In order to achieve such an object, the present invention according to claim 1 includes a pair of input waveguides and output waveguides, and the input waveguides and the output waveguides arranged at regular intervals. And a ladder interference filter comprising N (N; natural number of 3 or more) array waveguides connecting between the input waveguide and the output waveguide via the optical coupler. One or a plurality of low diffraction order regions having a certain optical path length difference in order from the first-stage array waveguide connected to the incident end side of the input waveguide; The optical path length difference of only i (M; natural number less than or equal to N−1, i; natural number greater than or equal to 1, M + i; N or less) array path length difference is determined as an M-th array waveguide. And (M + i) stage optical waveguide length difference is p times (p is a natural number) A high diffraction orders regions is equal to or coupled in series.

  The invention according to claim 2 is the wavelength tunable filter according to claim 1, wherein the light connected to the arrayed waveguide from the M-th stage to the (M + i) -th stage in the high diffraction order region. The optical coupling rate of the coupler is optimized.

  A third aspect of the present invention is the wavelength tunable filter according to the first or second aspect, further comprising an electrode having a certain length disposed on the waveguide between the optical couplers, Only the length of the electrode arranged in the high diffraction order region is set to p times the length of the electrode arranged in the low diffraction order region.

  A fourth aspect of the present invention is a wavelength tunable laser, the wavelength tunable filter according to any one of the first to third aspects, and an oscillation wavelength lock filter into which output light from the wavelength tunable filter is incident. And a semiconductor optical amplifier for amplifying output light from the oscillation wavelength locking filter.

  The invention according to claim 5 is the wavelength tunable laser according to claim 4, wherein the oscillation wavelength lock filter is a ring resonator.

  The invention described in claim 6 is a filter characteristic control method for a wavelength tunable filter, wherein in the wavelength tunable filter according to claim 3, when the center wavelength of the tunable filter is shifted to the short wavelength side, the input guide is provided. Current is injected into an electrode arranged in the waveguide, and current is injected into the electrode arranged in the output waveguide when the center wavelength of the variable filter is shifted to the long wavelength side.

  According to the present invention, there is provided a ladder interference type wavelength filter capable of narrowing the band, reducing the loss and increasing the wavelength variable range without increasing the number of waveguides of the arrayed waveguide and the diffraction order of the ladder interferometer. Can do. In addition, by using the ladder interference type wavelength filter according to the present invention as an oscillation wavelength selection element, it is possible to provide a wavelength tunable laser capable of oscillation wavelength stabilization, high output operation, and broadband wavelength tunable operation.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  A first embodiment of the present invention will be described with reference to FIG. FIG. 1 shows a configuration of a wavelength tunable laser according to the first embodiment. The wavelength tunable laser 100 includes a pair of an input waveguide 102 and an output waveguide 104, and optical couplers 130-1 to 130-N and 132-1 disposed in the input waveguide 102 and the output waveguide 104 at predetermined intervals. ˜132-N and N (N; 3 or more) connecting the input waveguide 102 and the output waveguide 104 via the optical couplers 130-1 to 130-N and 132-1 to 132-N (A natural number) array waveguides 140-1 to 140-N, and a ring resonator as an oscillation wavelength lock filter into which output light from the ladder interference type wavelength filter 120 is incident. 160, a semiconductor optical amplifier 180 that amplifies output light from the ring resonator 160, and an input end of the input waveguide 102 and an output end of the semiconductor optical amplifier 180, respectively. It continued the reflected mirror and a (reflective film) 190.

  Further, the ladder interference type wavelength filter 120 is constituted by two ladder interferometers having different diffraction orders, that is, a subordinate connection between the low diffraction order region 122 and the high diffraction order region 124.

When the number of array waveguides is N and the array waveguide at the incident end of the input waveguide 102 is the first-stage array waveguide, the array waveguide length in the low diffraction order region is (N−1) from the first stage. ) Sequentially increases by ΔS up to the stage array waveguide, and the transmittance is maximized at a wavelength (λ ₀ ) that satisfies the following equation.
λ ₀ = (n _{0 eff} ΔS) / m (1)

Here, n _eff is the effective refractive index of the optical waveguide, and m is the diffraction order of the ladder interferometer. On the other hand, the high diffraction order region is composed of a one-stage ladder interferometer, and the optical path length between the (N-1) -th array waveguide 140- (N-1) and the N-th array waveguide 140-N. If the difference is p × ΔS, the diffraction order of the ladder interferometer in this region has a diffraction order that is p times larger than the low diffraction order region (p is a natural number). Therefore, the wavelength (λ ₁ ) that maximizes the transmittance is expressed by the following equation.
λ ₁ = (n _eff pΔS) / (p × m) = (n _eff ΔS) / m (2)

  Therefore, as can be seen from the above equation, the center peak wavelengths of the low diffraction order region 122 and the high diffraction order region 124 in the ladder interference filter 120 coincide. In this case, the transmittance and FSR of the center peak wavelength are obtained from the low diffraction order region. On the other hand, a ladder interferometer having a high diffraction order serves to narrow the band near the center peak wavelength. The reflection mirror 190 is a reflection film provided for the optical feedback structure.

  Compared with the conventional wavelength tunable laser shown in FIG. 17 (for example, see Non-Patent Document 3), the ladder interference type wavelength filter has a low diffraction order region and a high diffraction order region.

FIG. 2 shows a calculation result of a transmission spectrum of the ring resonator 160 coupled in series with the ladder interference type wavelength filter 120 by a computer. The peak wavelength interval of the ring resonator 160 is 100 GHz. FIG. 2A shows the wavelength spectrum characteristics of the conventional ladder interference filter, and FIG. 2B shows the wavelength spectrum characteristics of the ladder interference filter in the wavelength tunable laser according to the first embodiment. Here, the diffraction order of the ladder interferometer is 40, the number of array waveguides is 15, and the optical coupling factor of the optical coupler used in the ladder filter is 0.85. However, for the ladder interference filter in the wavelength tunable laser according to the first embodiment, p in the high diffraction order region is set to 15. As shown by the solid lines in FIGS. 2A and 2B, the oscillation mode is determined by the wavelength spectrum characteristics of the ladder interference filter and the ring resonator connected in series, and the stability of the oscillation wavelength is determined by the ladder interference filter. It is determined by the suppression ratio between the ring resonance peak of the ring resonator that matches the center wavelength (λ ₀ ) and the adjacent ring resonance peak. As shown in FIG. 2B, in the case of the ladder interference filter 120 having the low diffraction order region 122 and the high diffraction order region 124, the 3-dB band is 1.65 nm, which is shown in FIG. Since the band can be narrowed compared with the conventional ladder interference filter (2.07 nm), the suppression ratio between adjacent ring resonance peaks is increased from 2.3 dB to 3.4 dB, and the oscillation wavelength can be stabilized. .

  However, since the ladder interference filter in the wavelength tunable laser according to the first embodiment has a high diffraction order region while maintaining the number of array waveguides, the element size naturally increases, and the optical path length increases. This causes a problem that the longitudinal mode interval of the laser resonator is also narrowed.

  Therefore, in order to overcome this problem, the number of array waveguides in the low diffraction order region must be reduced. In order for the ladder interference filter 120 in the wavelength tunable laser 100 according to the first embodiment to have the same element size as the conventional ladder interference filter having 15 array waveguides, for example, the number of array waveguides Is set to 8, p in the high diffraction order region is set to 8, or when the number of array waveguides is set to 12, p in the high diffraction order region is set to 4. That is, as the p in the high diffraction order region increases, the total number of array waveguides can be reduced, and as the number of array waveguides in the low diffraction order region increases, it approaches the conventional ladder interference filter characteristics.

  FIG. 3 shows the spectral characteristics of a conventional ladder interference filter (with the diffraction order of the ladder interferometer being 40 and the number of array waveguides being 15) and the same element size as that of the conventional ladder interference filter. The wavelength spectrum characteristic of the ladder interference type filter in the wavelength variable laser which concerns on 1st Embodiment is shown. FIG. 3A shows the wavelength spectrum characteristics of the ladder interference filter 120 in the wavelength tunable laser 100 according to the first embodiment when the number of arrayed waveguides is 14 and p in the high diffraction order region is 2. FIG. 3B shows the wavelength spectrum characteristics of the ladder interference filter 120 in the tunable laser 100 according to the first embodiment when the number of array waveguides is 11 and p in the high diffraction order region is 5. Show.

  As shown in FIG. 3A, it can be seen that the transmission band of the ladder interference filter of the present invention is slightly wider than that of the conventional ladder interference filter. This means that the broadening of the filter band due to the decrease in the number of array waveguides is more remarkable than the narrowing effect due to the high diffraction order region.

  Next, referring to FIG. 4, a ladder interference type wavelength filter 420 according to a second embodiment of the present invention will be described.

  In the ladder interference filter 120 in the wavelength tunable laser according to the first embodiment, it is effective to increase p in order to increase the narrowing effect by the high diffraction order region. The number of array waveguides decreases. In this case, as shown in FIG. 3B, the filter band is further widened, and it is difficult to narrow the band only by the high diffraction order region without increasing the element size.

  FIG. 4 shows the configuration of a ladder interference filter in which the position of the high diffraction order region is changed. Ladder interference filter 420 includes a pair of input waveguide 402 and output waveguide 404, and optical couplers 430-1 to 430-N and 432- arranged in input waveguide 402 and output waveguide 404 at predetermined intervals. 1 to 432-N, and N (N; 4 or more) connecting the input waveguide 402 and the output waveguide 404 via the optical couplers 430-1 to 430-N and 432-1 to 432-N (Natural number) array waveguides 440-1 to 440-N.

  When the number of array waveguides is N and the array waveguide at the incident end of the input waveguide 402 is the first-stage array waveguide, the first to M-th stages (M; 2 or more and N-1 or less) Natural number) and two low diffraction order regions 422 in which the waveguide lengths of the array waveguides from the (M + 1) -th stage to the N-th stage sequentially increase by ΔS, the M-th array waveguide, and the (M + 1) -th stage A high diffraction order region 424 in which a difference in optical path length from the array waveguide is p times ΔS is cascade-connected.

  The ladder interference filter 420 according to the second embodiment is a high diffraction order region that is subordinately connected (series coupled) to the low diffraction order region as compared with the ladder interference filter in the wavelength tunable laser according to the first embodiment. Only the position of is changed. Thereby, it is possible to obtain a narrow band characteristic without increasing the element size.

  In this case, the number of array waveguides, the diffraction order, and the optical coupling rate of the optical coupler can be the same as those of the ladder interference filter in the wavelength tunable laser according to the first embodiment shown in FIG.

  FIG. 5 shows the wavelength spectrum characteristics of the ladder interference filter shown in FIG. It can be confirmed that the band narrowing characteristic can be obtained by changing the connection position of the high diffraction order region. Such a large change in filter spectral characteristics is caused by a change in the transfer function of the ladder interference filter due to a change in the connection position in the high diffraction order region. In this case, the transmission band of the ladder interference filter changes depending on the connection position of the high diffraction order region. In order to obtain the best narrowband wavelength characteristics, the connection position of the high diffraction order region may be set to the center of the low diffraction order region, but there is a problem that the extinction ratio of the filter characteristics decreases as the bandwidth becomes narrower. .

  Therefore, the method of changing the connection position in the high diffraction order region causes a tradeoff between the narrowing of the filter and the extinction ratio.

  In the present embodiment, an example of a ladder interference filter having only one high diffraction order region has been described, but a plurality of high diffraction order regions can also be provided.

  Next, a ladder interference filter according to the third embodiment will be described with reference to FIGS. The ladder interference filter according to the third embodiment includes optical couplers 130-1 to 130-N and 132-1 to 132 of the ladder interference filter 120 in the wavelength tunable laser according to the first embodiment. It is characterized by optimizing the optical coupling rate (κ) of -N. As a result, it is possible to provide a ladder interference filter that can narrow the band or reduce the loss.

  In the ladder interference filter 420 in the tunable wavelength laser according to the second embodiment, although the narrow band is obtained by changing the connection position of the high diffraction order region 424, there is a trade-off with the extinction ratio of the ladder filter. It became a problem.

  FIG. 6 shows an optical coupler 130-1 to 130- (N-1) in the ladder interference filter according to the third embodiment, that is, the ladder interference filter 120 in the wavelength tunable laser according to the first embodiment. The optical coupling ratios (κ) of 132-1 to 132- (N-1) are set to 0.85, and the optical couplers 130-N and 132-N connected to the N-th array waveguide 140-N. The wavelength spectrum characteristics when only the optical coupling ratio (κ) is set to 0.5 are shown.

  That is, the connection position in the high diffraction order region is not changed, and only the optical coupling rate (κ) of the optical coupler is optimized. FIG. 6 shows wavelength spectrum characteristics when the number of array waveguides is 11 and p in the high diffraction order region is 5. As shown in FIG. 6, by optimizing the optical coupling rate (κ), the 3-dB band of the ladder interference filter is narrowed without increasing the element size as compared with the conventional ladder interference filter. It can be confirmed that

  Further, since the filter extinction ratio is increased by about 3 dB compared to the wavelength spectrum characteristic when the connection position of the high diffraction order region shown in FIG. 5 is changed, the filter was used for the wavelength tunable laser according to the first embodiment. In this case, the oscillation wavelength of the wavelength tunable laser can be stabilized.

  The optical coupling rate (κ) of the optical coupler used in the ladder interference type wavelength filter needs to be set so that the loss of the transmission wavelength peak is minimized. In general, a directional coupler or a multi-mode interference (MMI) coupler used as an optical coupler has a wavelength dependency in which a coupling characteristic varies depending on an input light wavelength.

  Therefore, as the number of optical couplers increases, the wavelength dependence becomes more prominent, and the loss of the transmission peak increases with the wavelength variable operation of the ladder interference filter, which causes the laser characteristics to deteriorate. Since the ladder interference filter according to various embodiments of the present invention can be narrowed using a relatively small number of optical couplers, it can be expected to reduce the insertion loss of filter elements and the wavelength dependence of spectral characteristics.

  In each of FIGS. 7A and 7B, calculation results of wavelength spectral characteristics of a conventional ladder interference filter using an MMI coupler as an optical coupler and a ladder interference filter according to the third embodiment are calculated by a computer. Respectively. In the numerical calculation shown in FIG. 7, in order to consider the wavelength dependence of the MMI coupler, the coupling coefficient of the MMI coupler is a function with respect to the wavelength. Further, the insertion loss of the MMI coupler was 0.2 dB / piece, and the propagation loss of the optical waveguide was 0.46 dB / cm.

  In the conventional ladder interference filter, the number of array waveguides is 15 and the diffraction order is 40. In the ladder interference filter according to the third embodiment, the number of array waveguides is 10, the diffraction order is 40, and the parameter p related to the high diffraction order region is 5.

  As shown in FIGS. 7A and 7B, since the optical coupling rate (κ) of the MMI coupler has wavelength dependence, the diffraction orders (m + 1) before and after the transmission band according to the desired diffraction order (m) and It can be seen that the transmission peak due to (m-1) decreases. In the case of the ladder interference filter according to the third embodiment, since the number of MMI couplers required for narrowing the band is small, the wavelength dependence of the spectral characteristics is weaker than that of the conventional ladder interference filter, and the center peak The advantage is that the wavelength transmittance (−5.5 dB) can be obtained higher than that of the conventional ladder interference filter (−6.9 dB).

  Therefore, if the ladder interference type wavelength filter according to the third embodiment is used for the wavelength tunable laser according to the first embodiment, the oscillation mode is stabilized by narrowing the band, the output power associated with the high output oscillation and the wavelength variable operation It becomes possible to reduce the wavelength dependence of the.

  The ladder interference filter according to the third embodiment can realize a narrowband wavelength spectrum characteristic by using not only a small number of array waveguides but also a small diffraction order as compared with a conventional ladder interference filter. Therefore, it is possible to expect an expansion of the wavelength variable range without deteriorating the oscillation characteristics of the wavelength variable laser according to the first embodiment.

  FIG. 8 shows the wavelength spectrum characteristics of a ladder interference filter having a diffraction order smaller than that of a conventional ladder interference filter. In this numerical calculation, the propagation loss of the optical waveguide, the insertion loss of the MMI coupler, and the wavelength dependence are taken into consideration as in FIG.

For the conventional ladder interference filter, the number of array waveguides is 15 and the diffraction order is 40. For the ladder interference filter of this example, the number of array waveguides was 8, the diffraction order was 30, and the parameter p in the high diffraction order region was 8. The optical coupling rate (κ) of the optical coupler was 0.85 at the center wavelength (λ ₀ = 1.55 μm). However, only the optical coupling rate (κ) of the optical coupler connected to the arrayed waveguide in the high diffraction order region was set to 0.5 (λ ₀ = 1.55 μm).

As shown in FIG. 8, the ladder interference filter of the present invention uses a relatively small diffraction order, and can achieve a transmission band comparable to that of a conventional ladder interference filter without increasing the element size. In addition, since the number of optical couplers is small, low loss and low wavelength dependent spectral characteristics are possible. Usually, the amount of change in transmission peak wavelength (Δλ) of a ladder interference filter due to a change in refractive index is expressed by the following equation.
Δλ = ΔnL _e / m (3)

Here, [Delta] n and L _e are each refractive index change amount and the refractive index change region length. That is, since the ladder interference filter of the present invention uses a diffraction order smaller than that of the conventional structure, a large Δλ can be obtained with respect to the same Δn, so that a broadband wavelength variable operation is possible. Further, since the FSR between the transmission bands is widened, there is an advantage that laser oscillation outside the desired wavelength region can be suppressed.

  FIG. 9 shows the wavelength spectrum characteristics of the ladder interference filter actually manufactured according to the third embodiment of the present invention. A solid line indicates the wavelength spectrum characteristic of the conventional ladder interference filter, and a broken line indicates the wavelength spectrum characteristic of the ladder interference filter according to the present invention. The production conditions of the prototype device are the same as those of the numerical calculation in FIG.

  As shown in FIG. 9, the ladder interference filter band of the present invention is observed to be 2.08 nm, which is almost the same as the band of the conventional ladder interference filter (2.04 nm), but the FSR of the transmission peak is relatively It can be widely confirmed that the transmittance of the central transmission peak is as high as 1.5 dB, and the wavelength dependence of the filter characteristics is small.

  As described above, the ladder interference filter described in this embodiment uses a diffraction order equivalent to 3/4 of the conventional ladder interference filter without increasing the element size, and has a narrow band, low loss, and low wavelength dependence. It has the feature that spectral characteristics are possible. That is, in the spectral characteristics shown in FIG. 9, the diffraction orders of the conventional ladder interference filter and the ladder interference filter according to the third embodiment are 40 and 30, respectively. When the diffraction order of the ladder interference filter according to the third embodiment is set to 90, a transmission band comparable to that of a filter having a conventional structure can be obtained. Therefore, the ladder interference filter of the present invention can always expand the FSR and the wavelength variable range by 4/3 times compared to the conventional ladder filter.

  Next, a fourth embodiment will be described with reference to FIG.

  The ladder interference filter 1020 according to the fourth embodiment has a high diffraction order region in which ladder interferometers having high diffraction orders are coupled in multiple stages. A ladder interference filter 1020 according to the fourth embodiment shown in FIG. 10 includes a pair of input waveguide 1002 and output waveguide 1004, and optical coupling arranged at predetermined intervals in the input waveguide 1002 and the output waveguide 1004. Between the input waveguide 1002 and the output waveguide 1004 via the optical couplers 1030-1 to 1030-N and 1032-1 to 1032-N, and the optical couplers 1030-1 to 1030-N and 1032-1 to 1032-N. It is composed of N (N: a natural number of 4 or more) arrayed waveguides 1040-1 to 1040-N connecting them.

  When the number of connected array waveguides of the array waveguide is N and the array waveguide at the incident end of the input waveguide 1002 is the first-stage array waveguide, the first to (N−2) -th stages are used. Only the difference in the optical path length of the arrayed waveguide from the (N−2) -th stage to the N-th stage is a constant difference in optical path length, and the low diffraction order region 1022 in which the waveguide length of the array waveguide sequentially increases by ΔS. , (N-2) and high diffraction order region 1024 that increases so that the difference between the waveguide lengths of the (N−2) -th stage and the N-th stage array waveguide is p times ΔS is connected in series (coupled in series). It has become.

  The ladder interference filter described in the first and second embodiments uses a structure in which a ladder interferometer having one stage of high diffraction order is connected in cascade, but has a high diffraction order as shown in FIG. Even when the ladder interferometer has a plurality of successive stages, it is possible to obtain narrow band characteristics as shown in FIG. In this case, there is an advantage that not only the band can be narrowed but also the extinction ratio of the filter can be increased as compared with a filter using a one-stage ladder interferometer.

  Next, referring to FIG. 11, a ladder interference type tunable filter according to a fifth embodiment of the present invention will be described.

  A ladder interference type tunable filter 1120 according to the fifth embodiment is a ladder interference type tunable filter provided with a current injection structure by electrodes 1110 and 1112 for wavelength tunable operation.

  A ladder interference type tunable filter 1120 shown in FIG. 11 includes a pair of input waveguides 1102 and output waveguides 1104 and optical couplers 1130-1 to 1130-1 arranged at predetermined intervals in the input waveguides 1102 and 1104. 1130-N and 1132-1 to 1132-N, and N connecting the input waveguide 1102 and the output waveguide 1104 via the optical couplers 1130-1 to 1130-N and 1132-1 to 1132-N This (N; natural number greater than or equal to 3) array waveguides 1140-1 to 1140-N, input waveguides 1102 and output waveguides 1104 and optical couplers 1130-1 to 1130-N and 1132-1 to 1132-N The electrodes 1110 and 1112 are arranged at intervals.

Further, the ladder interference type tunable filter 1120 has the layer structure of the epitaxial substrate shown in FIG. On an n-type InP substrate 1201, an n-doped InP layer 1202, a non-doped GaInAsP layer 1203 (band gap wavelength λ _g = 1.3 μm), a p-doped InP layer 1204, a p ⁺ -doped InP layer 1205, and a p ⁺ -doped InGaAs layer 1206 Are sequentially formed.

An AuZnNi electrode and an AuGeNi electrode are formed on the p ⁺ doped InGaAs layer 1206 and the n-type InP substrate 1201 shown in FIG.

In the wavelength variable operation of the ladder interference filter of this embodiment, the electrode lengths L ₁ and L ₂ shown in FIG. 11 must be optimized in order to keep the wavelength spectrum shown in FIG. 8 constant. Since the wavelength change amount (Δλ) of the ladder interference type wavelength filter by current injection is determined by the above equation (3), Δλ is inversely proportional to the diffraction order when the length of the electrode is constant.

In the case of the ladder interference filter according to the embodiment, a ladder interferometer having two different diffraction orders is cascade-connected. Accordingly, in order to keep the wavelength spectrum constant during the wavelength variable operation, it is necessary to make Δλ in the low diffraction order region and the high diffraction order region equal, and the electrode lengths L ₁ and L ₂ in the high diffraction order region 1124 are It is determined as follows:
Electrode length L ₁ = p × L ₁ (4) in the high diffraction order region
The length of the electrode in the high diffraction order region L ₂ = p × L ₂ , (L ₁ = L ₂ ) (5)

That is, by adjusting the length of the electrode in proportion to the change in the diffraction order, the wavelength spectrum can be kept constant during the wavelength variable operation. Ladder interference filter shown in FIG. 11, when current is injected into the refractive index control electrodes L _1, lambda ₀ is shifted to the short wavelength side, when current is injected into the refractive index control electrodes L _2, lambda ₀ is the long wavelength side Shift to.

FIGS. 13A and 13B show variable wavelength characteristics of the ladder interference type wavelength filter according to the fifth embodiment. As shown in FIG. 13A, unless the electrode lengths L ₁ and L ₂ in the high diffraction order region are optimized, the wavelength spectrum may be distorted and the laser oscillation may become unstable due to the wavelength variable operation. On the other hand, as shown in FIG. 13 (b), when the electrode length L _a and L _b optimizing in proportion to the diffraction order variation, can variable wavelength without distorting the wavelength spectrum, the laser oscillation becomes unstable Can be suppressed.

FIGS. 14A and 14B show the wavelength tunable characteristics of the ladder interference type filter experimentally manufactured based on the fifth embodiment. FIG. 14A shows a state of shift of the center wavelength λ ₀ when a conventional ladder interference filter is injected with a 40 mA current to the lower refractive index control electrode. It can be confirmed that the center wavelength λ ₀ has shifted to the 4.6 nm short wavelength side. On the other hand, FIG. 14B shows how the center wavelength λ ₀ is shifted when a current of 40 mA is injected into the refractive index control electrode in the lower part of the ladder interference filter produced based on the fifth embodiment. The center wavelength λ ₀ is shifted to the short wavelength side of 5.7 nm, the peak wavelength change amount is larger with respect to the same current injection amount, and it can be confirmed that the broadband wavelength can be varied as compared with the conventional ladder interference filter.

  In the present embodiment, the case where the wavelength of the ladder interference type wavelength filter is changed by current injection has been described. However, the wavelength of the ladder interference type wavelength filter can be changed by voltage application. Further, the wavelength of the ladder interference type wavelength filter can be changed by heat generated by current injection or voltage application.

1 is a configuration diagram of a wavelength tunable laser according to a first embodiment of the present invention. (A) is a figure which shows the transmission spectrum of the ring resonator connected in series with the ladder interference type | mold wavelength filter in the conventional ladder interference type | mold filter. (B) is a figure which shows the transmission spectrum of the ring resonator connected in series with the ladder interference type | mold filter which concerns on the 1st Embodiment of this invention. (A) is a figure which shows the wavelength spectrum characteristic of the ladder interference type filter which concerns on 1st Embodiment when the number of the arrayed waveguides of a low diffraction order area | region is 14 and p of a high diffraction order area | region is 2. FIG. . (B) shows the wavelength spectrum characteristics of the ladder interference filter according to the first embodiment of the present invention when the number of array waveguides in the low diffraction order region is 11 and p in the high diffraction order region is 5. FIG. It is a block diagram of the ladder interference type filter which concerns on 2nd Embodiment by which the high diffraction order area | region was cascade-connected between the low diffraction order area | regions. It is a figure which shows the wavelength spectrum characteristic of the ladder interference type filter which concerns on 2nd Embodiment. It is a figure which shows the characteristic of the ladder interference type filter which concerns on the 3rd Embodiment of this invention which optimized the optical coupling factor of the optical coupler. (A) is a figure which shows the wavelength spectrum characteristic which considered the wavelength dependence and insertion loss of the MMI optical coupler in the conventional ladder interference type filter. (B) is a figure which shows the wavelength spectrum characteristic which considered the wavelength dependence and insertion loss of an MMI optical coupler in the ladder interference type filter which concerns on the 3rd Embodiment of this invention. It is a figure which shows the characteristic of the ladder interference type | mold wavelength variable filter which concerns on the 3rd Embodiment of this invention. It is a figure which shows the wavelength spectrum characteristic of the ladder interference type filter prototyped with the ladder interference type filter which concerns on the 3rd Embodiment of this invention. It is a block diagram of the ladder interference type | mold wavelength variable filter which concerns on the 4th Embodiment of this invention. It is a block diagram of the ladder interference type | mold wavelength variable filter which concerns on the 5th Embodiment of this invention. It is a layer structure of the epitaxial substrate of the ladder interference type filter which concerns on the 5th Embodiment of this invention. (A) is a figure which shows the wavelength variable characteristic in case the electrode length is not optimized in the ladder interference type | mold wavelength variable filter which concerns on 5th Embodiment. (B) is a figure which shows the wavelength variable characteristic at the time of optimizing electrode length in the ladder interference type | mold wavelength variable filter which concerns on 5th Embodiment. (A) is a figure which shows the wavelength variable characteristic of the conventional ladder interference type filter. (B) is a figure which shows the wavelength variable characteristic of the ladder interference type | mold wavelength variable filter which concerns on the 5th Embodiment of this invention. It is a block diagram of the conventional wavelength tunable laser using a superperiod structure grating. It is a block diagram of the conventional wavelength tunable laser using AWG. It is a block diagram of the conventional wavelength tunable laser using a ladder interferometer and a ring resonator.

Explanation of symbols

100 Wavelength Tunable Lasers 120, 420, 1020, 1120, Ladder Interference Wavelength Filter 160 Ring Resonator 180, 1602 Semiconductor Optical Amplifier 190 Reflection Mirror 102, 402, 1002, 1102 Input Waveguide 104, 404, 1004, 1104 Output Waveguide 140-1 to 140-N, 440-1 to 440-N, 1040-1 to 1040-N, 1140-1 to 1140-N array waveguides 130-1 to 130-N, 132-1 to 132-N, 430-1 to 430-N, 432-1 to 432-N, 1030-1 to 1030-N, 1032-1 to 1032-N, 1130-1 to 1130-N, 1132-1 to 1132-N 122,422,1022,1122 Low diffraction order region 124,424,1024,1124 High diffraction order region 110 for refractive index control electrodes 1112 for refractive index control electrodes

Claims (6)

A pair of input waveguides and output waveguides, an optical coupler disposed at a certain interval in the input waveguides and the output waveguides, and between the input waveguides and the output waveguides via the optical couplers A tunable filter comprising a ladder interference filter composed of N (N: a natural number of 3 or more) array waveguides,
One or more low diffraction order regions having a certain optical path length difference in order from the first-stage array waveguide connected to the incident end side of the input waveguide;
Only the optical path length difference of the array waveguides of i pieces (M; natural number less than or equal to N−1, i; natural number greater than or equal to 1 and M + i; N or less) from the M-th stage is a constant optical path length difference. And a high diffraction order region in which the optical path length difference between the (M + i) -th array waveguide is p times (p is a natural number) is coupled in series.   2. The wavelength according to claim 1, wherein an optical coupling rate of the optical coupler connected to the array waveguide from the M-th stage to the (M + i) -th stage in the high diffraction order region is optimized. Variable filter. Further comprising a length of electrode disposed on the waveguide between the optical couplers;
3. The wavelength according to claim 1, wherein only the length of the electrode disposed in the high diffraction order region is set to p times the length of the electrode disposed in the low diffraction order region. Variable filter. The wavelength tunable filter according to any one of claims 1 to 3,
An oscillation wavelength locking filter on which output light from the wavelength tunable filter is incident;
A wavelength tunable laser comprising: a semiconductor optical amplifier that amplifies output light from the oscillation wavelength lock filter.   The wavelength tunable laser according to claim 4, wherein the oscillation wavelength lock filter is a ring resonator. The tunable filter according to claim 3,
When the center wavelength of the variable filter is shifted to the short wavelength side, current is injected into the electrode disposed in the input waveguide,
A method for controlling a filter characteristic of a wavelength tunable filter, wherein current is injected into an electrode disposed in the output waveguide when the center wavelength of the tunable filter is shifted to a long wavelength side.

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