JP2011086714A - Wavelength tunable laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent the increase of components even in a wavelength tunable laser using a wavelength selection filter having a sub-peak in addition to a main peak, and also to achieve stable single-mode oscillation, even in a transmission-type wavelength selection filter or a reflection-type wavelength selection filter. <P>SOLUTION: The wavelength selection filter is integrated in a gain chip, so as to select only the main peak. Thus, the stable single-mode oscillation is achieved. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a wavelength tunable laser. The tunable laser of the present invention is extremely useful when used for communication.

  The transmission capacity of optical communication is rapidly increasing, and wavelength division multiplexing (WDM) has been put into practical use as an inexpensive, high-speed, and large-capacity technology that meets this demand. WDM is a technology that uses multiple monochromatic lights (several tens to 100 wavelengths) with different frequencies by 50 GHz or 100 GHz simultaneously, and transmits different signals at each wavelength. In addition, this technology can increase the transmission capacity per fiber by several tens to 100 times, so that the cost of installing the fiber can be significantly reduced.

  Conventionally, a fixed wavelength semiconductor laser device has been used as such a WDM light source. When a fixed wavelength is used, it is necessary to manufacture a different semiconductor laser device for each wavelength. In order to manufacture different semiconductor laser devices for each wavelength, it is necessary to make different crystals, which is problematic in terms of cost. On the other hand, a wavelength tunable laser that can change the wavelength freely has been developed. For example, an example of a wavelength tunable laser using a structure that reflects with a wavelength selective reflecting mirror is disclosed in JP-A-2006-19514 (Patent Document 1). Since such a wavelength tunable laser is tunable within a wavelength range of about 40 nm, the types of semiconductor lasers to be manufactured can be reduced. For this reason, the laser itself can be provided at low cost and has become the main light source of WDM. Further, for WDM, from a practical point of view, there is an increasing demand for a wavelength tunable laser that is the same size as a fixed wavelength laser.

JP2006-19514

In general, a wavelength tunable laser has a gain portion made of a semiconductor material and a wavelength tunable filter that adjusts a laser oscillation wavelength. There are various types of wavelength tunable filters, but generally they have sub-peaks in addition to the main peak. For this reason, the oscillation based on the sub-peak may be induced to inhibit the single mode oscillation of the laser. For example, when the wavelength tunable filter is a liquid crystal etalon filter,
FSR _etalon = λ ² / 2nd
The filter characteristic appears repeatedly at the wavelength period determined by. Here, FSR _etalon is the wavelength interval of the transmittance peak of the liquid crystal etalon, λ is the wavelength, n is the refractive index of the liquid crystal, and d is the etalon thickness. In addition, as a supplement, FSR (Free Spectrum Range) is the wavelength interval of the transmittance peak (or reflectance peak) of the optical element. If this wavelength interval (that is, the repetition period) is narrower than the gain band of the gain section, two main and sub peaks may be included in the gain band, which inhibits the single mode oscillation of the laser described above.

  In order to avoid such difficulties and realize stable single mode oscillation, for example, a wavelength tunable laser having a structure in which light transmitted through a liquid crystal etalon filter is reflected by a wavelength selective reflecting mirror formed of a multilayer film is known. For example, it can be found in Japanese Patent Laid-Open No. 2006-19514 (Patent Document 1) described above.

  The basic configuration of such an example is shown in FIG. FIG. 1 is a plan view of the apparatus as viewed from above. The wavelength tunable laser in this example includes a gain chip 1 having an optical waveguide 2 and a gain section electrode 3, a lens 4, a liquid crystal etalon filter 5, and a wavelength selective reflecting mirror 6 composed of a multilayer film.

  The light from the gain chip 1 that is a light emitting element is transmitted through the liquid crystal etalon filter 5, the multilayer wavelength selective reflection mirror 6 reflects only the main mode wavelength, and the sub mode is transmitted. In this way, the multilayer wavelength selective reflector 6 and one end of the gain chip 1 constitute a laser resonator to realize single mode oscillation. FIG. 2 is a diagram illustrating the relationship between the main peak and sub peak of the liquid crystal etalon filter 5 and the reflectance of the multilayer film wavelength selective reflecting mirror 6. In FIG. 2, the vertical axis represents reflectance or transmittance, and the horizontal axis represents wavelength. The main peak of the transmittance of the liquid crystal etalon filter 5 is in the region where the reflectance of the multilayer wavelength selective reflector 6 is high, whereas the sub-peak of the transmittance is that of the region of the multilayer wavelength selective reflector 6 where the reflectance is low. It corresponds to the wavelength.

  However, in such a configuration, there are problems that the number of reflecting mirror parts for selecting a wavelength increases and the size of the laser device increases. In addition, when a reflective wavelength tunable filter is used for one of the optical resonators, it is necessary to dispose a bandpass filter inside the resonator. However, a bandpass filter that transmits broadband light having a wavelength of 40 nm or more is difficult to manufacture, and even if manufactured, the optical loss is large and the laser characteristics deteriorate. For this reason, when the reflection-type wavelength tunable filter has a sub peak in addition to the main peak, it is practically difficult to achieve single mode oscillation of the laser.

In order to solve the above-described problems, the present invention provides a wavelength tunable laser device that does not increase the number of components, can use a reflection-type wavelength tunable filter, and realizes stable single mode oscillation.
The basic configuration of the present invention includes a first reflecting mirror, a wavelength tunable filter, a gain chip that is a light-emitting element including a gain region for a laser beam, a wavelength selection filter region, and a second reflecting mirror. A tunable laser constituting an optical resonator,
The wavelength tunable filter is a filter that outputs light having a main peak at wavelength λm and a sub-peak whose intensity is weaker than the main peak at wavelength λs when light having a wavelength including wavelengths λm and λs is input. In addition, the wavelength selective filter region is a wavelength tunable laser characterized in that a reflectance R (λ) with respect to an incident wavelength has a relationship of R (λm)> R (λs).

  Here, it is also possible to use a wavelength tunable filter that also serves as the first reflecting mirror. Further, the second reflecting mirror of the optical resonator facing the first reflecting mirror can also take the form of a wavelength selection filter region included in the gain chip. For example, when the wavelength selective filter region is a region using a diffraction grating, the wavelength selective filter region can also be configured to serve as the second reflecting mirror. Conversely, when a diffraction grating assisted directional optical coupler is used for the wavelength selection filter region, a second reflecting mirror formed by a cleaved surface of the semiconductor is necessary. These are all one aspect of the present invention.

  The gist of the present invention will be described with reference to the examples shown in FIGS. FIG. 3 is a plan view of a rough configuration of the wavelength tunable laser of the present invention, FIG. 4 is a wavelength characteristic of the main and sub peaks of the transmittance of a liquid crystal etalon filter which is a specific example of the wavelength tunable filter, and the reflectance of the wavelength selective filter An example of the relationship is shown. In FIG. 4, the vertical axis represents reflectance or transmittance, and the horizontal axis represents wavelength. The basic configuration of this wavelength tunable laser is a liquid crystal etalon filter as a gain chip 31, a lens 34, and a wavelength tunable filter 35 (hereinafter, for ease of specific understanding, unless there is a special reason, the wavelength tunable filter 35 , Which is mainly referred to as a liquid crystal etalon filter as a specific example), and a first reflecting mirror 36. In the figure, reference numeral 32 denotes an optical waveguide, and reference numeral 33 denotes a gain section electrode. The area covered with the gain electrode becomes a gain section for laser oscillation. On the other hand, the region indicated by the optical waveguide 32 not covered with the gain electrode is the region of the wavelength selection filter. In the present invention, instead of the multilayer wavelength selective reflection mirror 6 in the example shown so far, as illustrated in FIG. 1, only the wavelength of the main peak of the liquid crystal etalon filter is selected and reflected by the wavelength selective filter 37, whereby the laser is selected. Constructs a resonator and realizes stable single mode oscillation. Accordingly, the wavelength selectivity required for the multilayer wavelength selective reflection mirror 6 so far is not necessary with respect to the first reflection mirror 36 of the present example. A mirror is sufficient. For example, a simple metal-coated total reflection mirror or the like. In the illustrated configuration, the wavelength selection filter 37 also serves as a second reflecting mirror that constitutes a laser resonator together with wavelength selection, and is integrated with the gain chip 31. (Hereinafter, in order to facilitate specific understanding, the wavelength selective filter 37 will be simply referred to as a semiconductor integrated wavelength selective filter as a specific example unless there is a particular reason.) In the example of FIG. 3, the semiconductor integrated wavelength selection filter as the wavelength selection filter 37 is integrated in the gain chip 31, and these are configured by separately forming a light emitting element called a gain chip and the wavelength selection filter. Is also possible. However, if the semiconductor integrated wavelength selection filter is integrated on the gain chip itself, it is easier to align the optical axes of both optical waveguides, and the optical axis alignment of both members, which is seen in separate cases, is unnecessary. It becomes. Therefore, from this point of view, the integrated form of both members is useful from the viewpoint of ensuring the characteristics of the laser device or manufacturing.

  In the present invention, as illustrated in FIG. 4, the reflectivity R (λ) of the semiconductor integrated wavelength selection filter 37 is R with respect to the main peak (wavelength λm) and sub-peak (wavelength λs) of the liquid crystal etalon filter 35. It is characterized by a relationship of (λm)> R (λs). That is, the wavelength of the sub peak (wavelength λs) of the liquid crystal etalon filter 35 is set to be outside the reflection band of the semiconductor integrated wavelength selection filter 37.

  A typical example of the wavelength selection filter 37 (that is, a semiconductor integrated wavelength selection filter) is a chirp distributed reflection Bragg reflector (Chirp Distributed Bragg Reflector), and the details of such members will be described later.

  According to the present invention, it is possible to provide a wavelength tunable laser device that realizes stable single mode oscillation without increasing the number of components even in a wavelength tunable laser using a wavelength selective filter having a sub peak in addition to a main peak.

FIG. 1 is a plan view of a wavelength tunable laser having a liquid crystal etalon filter, which is a typical example of a conventional wavelength tunable laser. FIG. 2 is a diagram showing an example of the transmittance spectrum of the liquid crystal etalon filter and the reflectance spectrum of the wavelength selective reflecting mirror of the wavelength tunable laser shown in FIG. FIG. 3 is a plan view of a wavelength tunable laser having a semiconductor integrated wavelength selection filter according to the present invention. FIG. 4 is a diagram showing the transmittance spectrum of the liquid crystal etalon filter of the example of FIG. 3 and the reflectance spectrum of the wavelength selection filter of the present invention. FIG. 5 is a diagram showing a transmission spectrum of the liquid crystal etalon filter. FIG. 6 is a cross-sectional view of a gain chip according to the first embodiment of the present invention. FIG. 7 is a diagram showing the x dependency of the diffraction grating pitch of the chirped DBR mirror (diffraction grating) used in the first embodiment of the present invention. FIG. 8A is a diagram showing an example of the reflection spectrum of the chirped DBR mirror (diffraction grating) of FIG. FIG. 8B is a diagram showing an example of a reflection spectrum of a chirped DBR mirror (diffraction grating) with L _DBR = 30 μm. FIG. 9 is a diagram illustrating an example of L _DBR dependence of reflectance flatness of a chirped DBR mirror (diffraction grating) within a wavelength band. FIG. 10 is a diagram illustrating an example of the relationship between the chirp amount and the diffraction grating length. FIG. 11 is a plan view of a tunable laser having a sampled DBR mirror as a semiconductor integrated wavelength selection filter according to the second embodiment of the present invention. FIG. 12 is a diagram showing the transmittance spectrum of the liquid crystal etalon filter and the reflectance spectrum of the wavelength selection filter used in the second embodiment of the present invention. FIG. 13 is a sectional view of a gain chip according to the second embodiment of the present invention. FIG. 14 is a sectional view of a sampled DBR mirror according to the second embodiment of the present invention. FIG. 15 is a diagram showing an example of the reflection spectrum of the sampled DBR mirror (diffraction grating) of FIG. FIG. 16 is a plan view of a wavelength tunable laser integrated with a semiconductor optical amplifier, which is a third embodiment of the present invention.

Prior to concretely illustrating the embodiments of the present invention, the main forms of the present invention are listed.
(1) The first form is
A wavelength tunable laser comprising an optical resonator having a first reflecting mirror, a wavelength tunable filter, a gain chip having a gain region and a wavelength selection filter region for laser light, and a second reflecting mirror. There,
The wavelength tunable filter is a filter that outputs light having a main peak at wavelength λm and a sub-peak whose intensity is weaker than the main peak at wavelength λs when light having a wavelength including wavelengths λm and λs is input. In addition, the wavelength selective filter region is a wavelength tunable laser characterized in that a reflectance R (λ) with respect to an incident wavelength has a relationship of R (λm)> R (λs).
(2) As the second form,
In the wavelength tunable laser according to item 1,
As the wavelength selection filter, one selected from the group consisting of a chirped DBR mirror, a sampled DBR mirror (Sampled Distributed Bragg Reflector), and a super structure DBR mirror (Super Structure Distributed Bragg Reflector) can be used.
The chirped DBR mirror uses a chirped diffraction grating as a reflecting mirror by DBR, the sampled DBR mirror forms a uniform diffraction grating formed region and a region without a diffraction grating repeatedly, and the superstructure DBR mirror is short. And the like using a superstructure diffraction grating in which gratings are discretely produced at equal intervals. It is also possible to use a diffraction grating assisted unidirectional optical coupler. In the wavelength selection filter using the diffraction grating exemplified here, the wavelength selection filter and the reflecting mirror of the optical resonator are also used.
(3) The third form is
In the wavelength tunable laser according to item 2,
The diffraction grating length L _DBR and the chirp amount Δλc of the chirp DBR mirror are
−4 μm < _LDBR − (0.0037 × Δλc 2 − 0.84 × Δλc + 56) <+ 4 μm
as well as,
_{-2μm <L DBR - (0.0045 ×} Δλc 2 - 1.1 × Δλc + 95) <+ 2μm
The wavelength tunable laser is characterized in that the relationship is removed.
(4) The fourth form is
In the wavelength tunable laser according to item 2,
The diffraction grating length L _DBR and the chirp amount Δλc of the chirp DBR mirror are
L _DBR − (0.0037 × Δλc 2 − 0.84 × Δλc + 56)> + 4μm
-2μm> L _DBR − (0.0045 × Δλc 2 − 1.1 × Δλc + 95)
It is a wavelength tunable laser characterized by satisfying the relationship of satisfying both.

  The gain chip 31 includes at least a first cladding region, a core portion of an optical waveguide, and a stack of second cladding regions on an upper portion of a semiconductor substrate serving as a gain chip base. On the gain chip substrate, both regions of the optical waveguide 32 and the wavelength selection filter 37 serving as the second reflecting mirror are provided in the direction along the optical axis, and further formed at least above the optical waveguide 32. The gain portion electrode 33 has a back electrode on the back surface of the gain chip base. By injecting current from the gain part electrode 33, light emission in the gain part is enabled.

  The semiconductor integrated wavelength selection filter 37, which is the wavelength selection filter described above, includes a chirped distributed reflection type Bragg reflector (chirped DBR mirror), a sampled DBR mirror, a superstructure DBR mirror, a diffraction grating assisted directional light coupling A region such as a vessel is generally configured to have at least a first cladding region, a core portion of an optical waveguide, and a stack of second cladding regions on an upper portion of a semiconductor substrate. A diffraction grating, a superstructure, or the like is formed in a direction along the optical axis so that a diffraction effect or the like is generated for the light propagating through the optical waveguide portion.

  The wavelength tunable filter may be a transmission wavelength tunable filter such as a liquid crystal etalon filter or a vernier glass etalon filter, or may be a reflective liquid crystal filter.

Hereinafter, specific embodiments of the present invention will be described.
A first embodiment of the present invention will be described with reference to FIGS. The first embodiment is an example of a wavelength tunable laser having a chirped DBR mirror operating as a semiconductor integrated wavelength selection filter operating in a C band band (wavelengths 1530 nm to 1570 nm).
That is, as shown in FIG. 3, the wavelength tunable laser of this example includes a gain chip 31, which is a light emitting element, a lens 34, a liquid crystal etalon filter 35, and a total reflection mirror 36. Although the description of FIG. 3 has been made previously, the gain chip 31 has a gain section and a semiconductor integrated wavelength selection filter section. Reference numeral 32 denotes a core portion of the optical waveguide. The region covered with the gain electrode 33 is the gain portion, and the other region of the optical waveguide is the semiconductor wavelength selection filter region 37. Depending on the type of the semiconductor wavelength selection filter region, the wavelength selection filter unit electrode 38 may be disposed above the semiconductor wavelength selection filter region. If the reflectance is high in the entire range where the wavelength is desired, such as a chirp distributed reflection type Bragg reflector (chirp DBR mirror), no electrode is required. On the other hand, when sampled DBR mirrors, superstructure DBR mirrors, diffraction grating assisted directional optical couplers, etc., where the reflection or filter band is narrower than the range where the wavelength is desired to be tunable, it is necessary to adjust by current injection. An electrode is required. A cross-sectional structure diagram of the gain chip 31 will be described later. A laser resonator is formed between the total reflection mirror 36 and the semiconductor integrated wavelength selection filter 37 that also serves as the second reflection mirror, while the liquid crystal etalon filter 35 controls the oscillation wavelength.

  In order to adjust the temperature of the gain chip 31 and stabilize the optical system, the gain chip 31, the lens 34, the wavelength tunable filter 35, the reflecting mirror 35, and the like are usually mounted on a Peltier element or the like.

  For the liquid crystal etalon filter 35, a usual configuration is sufficient. In other words, the liquid crystal etalon filter changes the optical path length of the inter-reflector cavity by filling the liquid crystal in the inter-reflector cavity constituting the Fabry-Perot etalon and applying a voltage to the liquid crystal to change the refractive index. This is a tunable filter that can be made to operate. A first transparent electrode and a second transparent electrode for applying a voltage to the liquid crystal layer; a first high-reflectivity mirror that reflects light of a predetermined wavelength; and a second high-reflectance mirror ing. Since the configuration of the liquid crystal etalon filter itself is a usual one, its detailed illustration is omitted. The characteristics of the liquid crystal etalon filter of this example are as follows: the reflectivity of the reflector is 95.5%, the liquid crystal material is 12 μm thick, and the refractive index is 1.7 when no voltage is applied. FIG. 5 shows a transmission spectrum of the liquid crystal etalon filter 35. The horizontal axis represents wavelength and the vertical axis represents transmittance. There is a transmission peak at 1570 nm (= λm). By applying a voltage to the liquid crystal material, the transmission peak is shifted by a short wavelength, and a wavelength variable operation is realized. However, here, there is also a peak at a wavelength of 1512 nm (= λs) on the short wavelength side by Δλ = 58 nm. This peak becomes a factor that hinders the single mode oscillation of the wavelength tunable laser. The present invention relates to a countermeasure for these two peaks. The details will be described later.

  Next, a cross-sectional view of the gain chip of this example is shown in FIG. An InGaAsP active layer 62 (a plurality of InGaAsP active layers having an optical confinement structure, hereinafter simply referred to as an InGaAsP active layer 62) is applied to a substrate 61 to be n-type InP. It is formed by vapor phase growth (MOCVD: Metal Organic Chemical Vapor Phase deposition). Specifically, the detailed structure of the InGaAsP active layer 62 in this example includes an n-type InGaAsP optical confinement layer, an InGaAsP strained multiple quantum well layer, and a p-type InGaAsP optical confinement layer from a layer close to the n-type InP substrate 61. Do it. Since the InGaAsP active layer itself has a general configuration, the detailed internal structure is omitted in the figure and is shown as a single layer. As a specific structure of the active layer of the gain chip, a further layer that can be used for oscillation of the semiconductor laser can be used as needed.

  Next, a part of the InGaAsP active layer 62 is removed by reactive ion etching (RIE) using the silicon nitride layer as a mask, and a core layer made of bulk InGaAsP is formed in the removed region. 63 (composition wavelength: 1.3 μm) is selectively grown using the MOVPE method. Subsequently, a resist film is formed on the core layer 63 made of bulk InGaAsP, and a diffraction grating pattern is formed by electron beam exposure. Then, using the resist on which the diffraction grating pattern is formed as a mask, the diffraction grating layer 64 is formed on the bulk InGaAsP core layer 63.

  Subsequently, a clad layer 65 made of p-type InP and a contact layer 66 made of p-type InGaAs are grown as a common clad on the entire surface of the wafer by using the MOVPE method.

  A silicon dioxide film is coated on the InP wafer having such a multilayer structure so as to cover the main part of the device thus far formed, that is, the InGaAsP active layer 62, 63 InGaAsP core layer, and the diffraction grating layer 64, and a protective mask. To do. Using this silicon dioxide mask, the side wall along the optical axis of the main part of the device is etched to the n-type InP substrate. For the etching, for example, dry etching such as reactive ion etching (RIE) using chlorine-based gas, wet etching using bromine-based solution, or the like, or a combination of both methods may be used.

Subsequently, semi-insulating InP is regrown on the side wall along the optical axis of the main part of the device by using the MOVPE method.
Further, the contact layer 66 made of p-type InGaAs other than on the InGaAsP active layer 62 is removed, and a silicon dioxide film 67, a gain part electrode 68, and an electrode 69 are formed on the wafer surface, and the wafer is completed.

  Thereafter, after forming both end faces of the gain chip 31 by cleavage, antireflection coating films 70 and 71 are formed on these end faces. Thus, the gain chip in this example is completed.

  Next, a structure corresponding to the semiconductor integrated wavelength selection filter 37 will be described. As described above, the semiconductor integrated wavelength selection filter only needs to have a relationship of R (λm)> R (λs). From the standpoint of single wavelength selection, λm and λs can be reversed, but from the relationship of the peak value, R (λm)> R (λs) is practically useful.

  On the other hand, if the reflection band of the semiconductor integrated wavelength selection filter 37 includes a wavelength variable range (1530 nm to 1570 nm in this example), single mode oscillation is performed over the entire wavelength variable width without adjusting the wavelength of the semiconductor integrated wavelength selection filter. To do. A broadband reflector having such a reflection band can be realized using, for example, a chirped diffraction grating.

The chirped diffraction grating has a configuration in which the pitch of the diffraction grating is continuously changed in the propagation direction of the core of the optical waveguide, and the wavelength reflected in the propagation direction can be continuously changed. FIG. 7 shows an example of x (diffraction grating length) dependence of the diffraction grating pitch in the chirped diffraction grating. That is, FIG. 7 is a diagram showing the relationship between the diffraction grating pitch (vertical axis) and x (horizontal axis) in a portion corresponding to the region having the chirped diffraction grating of FIG. The origin of x is shown as x = 0 in FIG. The diffraction grating length L _DBR was 21 μm, and the diffraction grating pitch was linearly changed from 235.3 nm to 248.4 nm. The Bragg wavelengths at this time are 1506 nm and 1590 nm, respectively. The chirp amount Δλc at this time is defined as 84 nm from the wavelength difference of the Bragg wavelength. FIG. 8A shows a reflection spectrum of the chirped diffraction grating illustrated in FIG. Calculated from the commonly known coupled wave equation. It has a substantially flat reflection characteristic in the C band (wavelength 1530nm-1570nm), while the reflectance is 1% or less at λs = 1512nm, and the sub peak of the liquid crystal etalon filter 35 has an extremely small reflectance. Laser oscillation at a wavelength can be suppressed.

  With the above-mentioned wavelength tunable laser, single mode oscillation could be achieved in the C band band wavelength range of 1530 nm to 1570 nm.

  The difference between the reflectances R (λm) and R (λs) is preferably about 10% or more for practical use.

Next, the relationship between the chirp amount Δλc and the diffraction grating length L _DBR in the chirped diffraction grating will be described in detail. More specifically, the reflectance flatness of chirped grating is dependent largely on the diffraction grating length L _DBR, it should be noted the relationship between the chirp amount Δλc the diffraction grating length L _DBR.

FIG. 8B shows a reflection spectrum when L _DBR = 30 μm and the diffraction grating pitch is linearly increased from 235.3 nm to 248.4 nm (chirp amount Δλc = 84 nm). Similar to FIG. 8A, calculation was performed from a commonly known coupled wave equation. In the example of FIG. 8B, the reflection band is the same as in FIG. 8A (L _DBR = 21 μm), but the unevenness of the reflectance in the C band band is large. As a result, the fluctuation of the light output is large and the laser becomes unstable. Thus, the reflectivity flatness of the chirped diffraction grating depends greatly on L _DBR .

FIG. 9 shows the L _DBR dependence of the reflectance flatness within the wavelength band. FIG. 9 shows an example when the chirp amount Δλc = 84 nm. The reflectance flatness within the wavelength band is defined by the ratio between the maximum value and the minimum value of the reflectance within the wavelength band. As the reflectivity flatness is closer to 1, the reflectivity is flat and stable laser oscillation can be expected.

At present, from a practical viewpoint of a laser device, for example, as a standard for stable laser oscillation, a flatness of reflectance in a wavelength band> 0.4 is targeted. In this case, as shown in FIG. 9, L _DBR must satisfy 0 μm ≦ L _DBR ≦ 8 μm (a), 16 μm (b) ≦ L _DBR ≦ 28 μm (c), and 32 μm (d) ≦ L _DBR . Conversely, it is necessary to make a chirped diffraction grating except for 8 μm <L _DBR <12 μm and 28 μm <L _DBR <32 μm.

This range depends on the chirp amount. FIG. 10 shows the relationship between the chirp amount and the diffraction grating length. Similar to FIG. 9, the relationship between each chirp amount and the diffraction grating length L _DBR was obtained over the chirp amount shown in FIG. 10 ranging from 50 nm to 110 nm. The results are summarized in FIG. For example, in FIG. 9, the values a, b, c, and d corresponding to the reflectance flatness> 0.4 correspond to the diffraction grating length L _DBR at the chirp amount Δλc = 84 nm in FIG. In FIG. 10, the chirp amount and the length of the diffraction grating are numerically described as examples of a, b, c, and d. Similar calculations were performed for other chirp amounts Δλc = 62, 74, 94, and 104 nm, and the range of FIG. 10 was determined.

In order to achieve the reflectance flatness in the wavelength band> 0.4, a chirped diffraction grating in the region excluding the regions A and A ′ is preferable. The range corresponds to region A,
−4 μm <LDBR − (0.0037 × Δλc ² − 0.84 × Δλc +56) <+ 4 μm
And the area A ′,
−2 μm <LDBR − (0.0045 × Δλc ² − 1.1 × Δλc +95) <+ 2 μm
This is the area excluding. The equation was obtained by fitting the points obtained from the above five chirp amounts.

  That is, as seen in FIG. 9, the area excluding the areas A and A ′ is an area where the reflectance flatness within the wavelength band exceeds 0.4.

More preferably, it is a chirped diffraction grating in the region B in FIG. That is, the range is
L _DBR − (0.0037 × Δλc ² − 0.84 × Δλc + 56)> + 4μm
And _{-2μm> L DBR - (0.0045 ×} Δλc 2 - 1.1 × Δλc + 95)
It becomes an area where both are compatible. The equation was obtained by fitting the points obtained from the above five chirp amounts.

  Although it can be sufficiently judged from the example of FIG. 9, the length of the diffraction grating is not very short, is suitable for processing, and the reflectance flatness within the wavelength band sufficiently exceeds 0.4, and Higher reflectance flatness within the wavelength band can be ensured than other diffraction grating length regions.

  Even if the above-described wavelength selection filter is replaced with a reflective liquid crystal filter, an etalon filter, or a vernier glass etalon filter, a wavelength tunable laser equivalent to the previous examples can be obtained. Each wavelength tunable laser can obtain the same characteristics as the above-described example.

  A second embodiment of the present invention will be described with reference to FIGS. This embodiment is a wavelength tunable laser having a sampled DBR mirror operating as a semiconductor integrated wavelength selection filter operating in the C band (wavelength 1530 nm to 1570 nm). The difference from the first embodiment is that the wavelength selective filter is a sampled DBR mirror and that an electrode is provided in the semiconductor integrated wavelength selective filter section.

FIG. 11 shows a wavelength tunable laser. 3 is substantially the same as that of the first embodiment, except that a semiconductor integrated wavelength selection filter unit electrode 117 is added. A laser resonator is formed between the total reflection mirror 116 and the semiconductor integrated wavelength selection filter 117, while the liquid crystal etalon filter 115 and the semiconductor integrated wavelength selection filter 117 control the oscillation wavelength. Since other configurations are the same as those in the embodiment, detailed description thereof is omitted.
FIG. 12 shows the relationship between the transmittance of the liquid crystal etalon filter and the reflectance of the sampled DBR mirror. In FIG. 12, the horizontal axis represents wavelength and the vertical axis represents reflectance or transmittance. There is a transmission peak of the liquid crystal etalon filter at 1570 nm (= λm). On the other hand, the liquid crystal etalon filter has a peak at a wavelength of 1512 nm (= λs), which inhibits single mode oscillation of the laser. For this reason, in this example, the reflectance R (λ) of the sampled DBR mirror has a relationship of R (λm)> R (λs). As a result, the sub mode of the liquid crystal etalon filter 115 is suppressed, and stable single mode oscillation of the laser can be realized.

  FIG. 13 shows a cross-sectional structure of an example of the gain chip of this example. The structure is the same as that of FIG. 6 except that the diffraction grating 134 is configured as a sampled diffraction grating, the region is a sampled DBR reflector, and the semiconductor integrated wavelength selection filter unit electrode 142 is added. Structure. That is, reference numeral 131 is an n-type InP substrate, reference numeral 132 is an InGaAsP active layer (consisting of a plurality of layers having an optical confinement layer as in the above example), reference numeral 135 is a p-type InP cladding layer, and reference numeral 136 is a p-type InGaAs. Contact layer, 133 is an InGaAsP core layer, 134 is a diffraction grating layer, 137 is a silicon dioxide film, 138 is a gain electrode, 139 is a back electrode, 140 and 141 are non-reflective coating films, 142 is It is a semiconductor integrated wavelength selection filter part electrode.

FIG. 14 is an enlarged partial sectional view of the sampled DBR mirror, and FIG. 15 shows its reflection spectrum. FIG. 14 is an enlarged view of the corresponding area of FIG. As shown in FIG. 14, L1 represents the width of the region of the provided diffraction grating, and Λs represents the period of the diffraction grating. In this example, as shown in the figure, diffraction gratings for five periods were formed with L1 = 5 μm and Λs = 56 μm. As a result, as shown in FIG. 15, a reflectance having a repetitive reflection peak of FSR _DBR = 6.1 nm was obtained. As a result, a relationship of R (λm)> R (λs) as shown as a model diagram in FIG. 12 was obtained, and stable single mode oscillation was realized. Here, FSR _DBR is the wavelength interval of the transmittance peak of the sampled DBR reflector. In addition, by controlling the voltage applied to the liquid crystal etalon filter 115 and controlling the injection current to the semiconductor integrated wavelength selection filter 117, the oscillation wavelength of the laser could be made variable. In this example, single mode oscillation was achieved in the wavelength range of 1530 nm to 1570 nm.

  Here, the sampled DBR mirror is shown as the semiconductor integrated wavelength selection filter, but instead of this, the wavelength using a DBR mirror having a repetitive reflectance such as a superstructure DBR mirror or a phase shift DBR mirror is used. A variable laser could be created. Each wavelength tunable laser according to the invention can obtain the same characteristics as the above-described example.

  Further, even if the wavelength selection filter is replaced with a reflective liquid crystal filter, an etalon filter, or a vernier glass etalon filter, a wavelength tunable laser equivalent to the previous examples can be obtained. Each wavelength tunable laser can obtain the same characteristics as the above-described example.

  In the present invention, as exemplified in the above two embodiments, the semiconductor integrated wavelength selection filter has a repetitive reflectance such as a chirped DBR mirror, a sampled DBR mirror, a superstructure DBR mirror, or a phase shift DBR mirror. By using one person such as a DBR mirror, characteristics equivalent to these can be obtained. On the other hand, one of the reflection type liquid crystal filter, etalon filter, vernier type glass etalon filter, etc. can be used as the wavelength selection filter. Of course, even if these various semiconductor integrated wavelength selection filters and various wavelength selection filters are used in combination, a wavelength tunable laser equivalent to the above examples can be obtained.

  In the above two embodiments, an example of application to the C band band (wavelengths 1530 nm to 1570 nm) is shown. However, the present invention is applicable to other band bands such as 1570 nm to 1610 nm, or 1270 nm to 1330 nm. Needless to say, the same applies.

  A third embodiment of the present invention will be described with reference to FIG. This embodiment is a wavelength tunable laser operating in a C band band (wavelengths from 1530 nm to 1570 nm) in which a semiconductor optical amplifier (SOA) is integrated. The difference from the first embodiment is that SOA is integrated in the gain chip 161.

  A laser resonator is formed between the all-wavelength selective reflection mirror 166 and the semiconductor integrated wavelength selective filter 167, and the liquid crystal etalon filter 165 and the semiconductor integrated wavelength selective filter 167 control the oscillation wavelength. Reference numeral 162 denotes an optical waveguide, reference numeral 163 denotes a gain part electrode, and reference numeral 168 denotes an SOA part electrode. Therefore, the portion where the SOA electrode is disposed is the SOA region with respect to the optical waveguide.

  In this example, the cleaved end face is not used for laser oscillation, and the diffracted light by the diffraction grating is equivalent to the reflected light. Therefore, as in the conventional example, the SOA that was not possible when the front reflecting mirror was the cleaved end face was used. It became possible to accumulate. Light can be amplified by this SOA.

  This group length variable laser achieved a single mode oscillation that was amplified by 10 dB compared to a laser with no SOA. The wavelength range is a wavelength range of 1530 nm to 1570 nm.

  Here, only the case where the SOA is integrated is shown, but a configuration in which the optical modulator is integrated can be taken.

1: gain chip, 2: optical waveguide, 3: gain section electrode, 4: lens, 5: liquid crystal etalon filter, 6: multilayer wavelength selective mirror, 7: submount, 8: Peltier cooler, 9: optical fiber, 31: gain chip, 32: optical waveguide, 33: gain section electrode, 34: lens, 35: liquid crystal etalon filter, 36: total reflection mirror, 37: semiconductor integrated wavelength selection filter, 38: wavelength selection filter section electrode, 39: Submount, 40: Peltier cooler, 41: optical fiber, 61: n-type InP substrate, 62: InGaAsP active layer, 63: InGaAsP core layer, 64: diffraction grating layer, 65: p-type InP cladding layer, 66: p-type InGaAs contact layer, 67: silicon dioxide film, 68, gain part electrode, 69: electrode, 70, 71: non-reflective coating film, 111: gain chip, 112: optical waveguide, 113: gain part electrode, 114: lens, 11 5: Liquid crystal etalon filter, 116: All wavelength selective reflector, 117: Semiconductor integrated wavelength selective filter, 118: Wavelength selective filter part electrode, 119: Submount, 120: Peltier cooler, 121: Optical fiber, 131: n-type InP Substrate, 132: InGaAsP active layer, 133: InGaAsP core layer, 134: diffraction grating layer, 135: p-type InP cladding layer, 136: p-type InGaAs contact layer, 137: silicon dioxide film, 138, gain electrode, 139: Electrode, 140, 141: Non-reflective coating film, 142: Semiconductor integrated wavelength selection filter part electrode
161: Gain chip, 162: Optical waveguide, 163: Gain part electrode, 164: Lens, 165: Liquid crystal etalon filter, 166: Total reflection mirror, 167: Semiconductor integrated wavelength selection filter, 168: SOA part electrode, 169: Submount 170: Peltier cooler, 171: optical fiber.

Claims (7)

A wavelength tunable laser comprising an optical resonator having a first reflecting mirror, a wavelength tunable filter, a gain chip having a gain region and a wavelength selection filter region for laser light, and a second reflecting mirror. There,
The wavelength tunable filter is a filter that outputs light having a main peak at wavelength λm and a sub-peak whose intensity is weaker than the main peak at wavelength λs when light having a wavelength including wavelengths λm and λs is input. And the wavelength selective filter region has a relationship of reflectivity R (λ) with respect to an incident wavelength such that R (λm)> R (λs). In the wavelength tunable laser according to claim 1,
A wavelength tunable laser, wherein the wavelength selection filter is one selected from the group consisting of a chirped DBR mirror, a sampled DBR mirror, and a superstructure DBR mirror. The tunable laser according to claim 2,
The diffraction grating length L _DBR and the chirp amount Δλc of the chirp DBR mirror are
-4μm < _LDBR- (0.0037 x Δλc 2-0.84 x Δλc + 56) <+ 4μm
as well as,
_{-2μm <L DBR - (0.0045 ×} Δλc 2 - 1.1 × Δλc + 95) <+ 2μm
A wavelength tunable laser characterized by the relationship excluding. The tunable laser according to claim 2,
The diffraction grating length L _DBR and the chirp amount Δλc of the chirp DBR mirror are
_{L DBR - (0.0037 × Δλc 2} - 0.84 × Δλc + 56)> + 4μm
And
_{-2μm> L DBR - (0.0045 ×} Δλc 2 - 1.1 × Δλc + 95)
A wavelength tunable laser characterized by satisfying the relationship of satisfying both. In the wavelength tunable laser according to claim 1,
A wavelength tunable laser, wherein the wavelength selective filter is a diffraction grating assisted directional coupler. In the wavelength tunable laser according to claim 1,
A wavelength tunable laser, wherein the wavelength selection filter is a reflective liquid crystal filter, an etalon filter, or a vernier glass etalon filter. In the wavelength tunable laser according to claim 1,
A wavelength tunable laser, wherein a gain amplifier is integrated with a semiconductor amplifier or a modulator.

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