JP2004317783A - Gain clamping optical amplifier - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a gain clamping optical amplifier which is small-sized, has high performance, and is inexpensive. <P>SOLUTION: The gain clamping optical amplifier is provided with: an SOA device 4 whose input side and output side are connected to an optical waveguide 2; and UV gratings 5 and 6 which are provided at parts of respective waveguides 2 and reflect light of a specified wavelength. At least the UV grating 6 provided at a light output end side is composed of a blazed grating which returns only part of a reflected light wave to the SOA device 4 and radiates part of the rest. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical amplifier used in optical communication and the like, and particularly to a gain clamp optical amplifier used for linear amplification and optical signal processing.
[0002]
[Prior art]
As one technique for obtaining a linear operation of an optical amplifier, a so-called “gain clamp” technique has been proposed in which the gain is clamped by intentionally oscillating the optical amplifier with a laser. In particular, since a semiconductor optical amplifier (SOA) has a large nonlinearity and a high response speed, signal waveform deterioration occurs when a modulated signal is transmitted, and gain fluctuation and crosstalk occur when amplifying wavelength-multiplexed light. It is known that such a problem occurs.
[0003]
In order to solve these problems, a device configuration called “gain clamp SOA” to which the above-described gain clamp technology is applied has been reported. Contrary to linearization, in the “gain clamp SOA”, since extremely large nonlinearity is obtained near the oscillation threshold, application to optical signal processing as a wavelength conversion, an optical logic gate, or the like is also expected. .
[0004]
10 is described in reference 1 (M. Bachmann, et al., "Polarization-insensitive clamped-gain SOA with integrated spot-size converters and related licenses for the DBR gratings for the first time as a whole). No. 22, pp. 2076-2078, 1996.) is a top view schematically showing a configuration example of a conventional gain clamp SOA. As shown in the figure, in this structure, an active region 01 for amplifying a light wave and optical waveguides provided on both sides thereof, on which Bragg gratings 02a and 02b having a reflection center wavelength λc and a reflectance Rc are formed, are semiconductors. It is formed collectively.
[0005]
According to such a structure, since a resonator is formed between the Bragg gratings 02a and 02b on both sides, the balance between the reflectance on both sides of the Bragg grating and the amplification gain, and the phase condition (vertical direction) when the resonator makes one round. Under the conditions of (mode condition) and the like, an oscillation operation can be obtained at a wavelength approximately in the vicinity of λc. Generally, at the time of laser oscillation, the carrier density of the active region 01 is kept constant, and thus, in such a gain clamp SOA, a so-called linear amplification operation in which the gain is kept constant under predetermined operating conditions can be obtained. .
[0006]
Since the oscillation threshold is generally determined by the balance between the grating reflectance Rc and the gain, a desired gain spectrum can be obtained after clamping by appropriately setting the reflection wavelength λc and the reflectance Rc of the Bragg gratings 02a and 02b. be able to.
[0007]
[Non-patent literature]
M. Bachmann, et al. , “Polarization-insensitive clamped-gain SOA with integrated spot-size converter and DBR gratings for WDM applications, 1.55 μm newsletter. 32, no. 22 pp. 2076-2078, 1996.
[0008]
[Problems to be solved by the invention]
By the way, in the gain clamp SOA, as shown in FIG. 11, the oscillation light is output with a large output intensity together with the signal light to be amplified. Such high-level oscillating light becomes a large noise at the time of reception at the end of the transmission line, so it must be removed before the photodetector, as well as saturating the optical amplifier after the gain clamp SOA, It also causes various problems in optical transmission lines, such as inducing unnecessary nonlinear effects in optical fibers.
[0009]
Therefore, generally, a high-performance optical filter is provided immediately after the gain clamp SOA, and this is removed. For this reason, when the conventional gain clamp SOA is actually used, there are problems such as an increase in the size of the entire device and an increase in cost.
[0010]
An object of the present invention is to provide a compact, high-performance, and inexpensive gain-clamped optical amplifier in view of the above conventional technology.
[0011]
[Means for Solving the Problems]
The configuration of the gain-clamped optical amplifier according to the present invention that solves the above-described problems is as follows:
An optical amplifier having an optical input end and an optical output end; an optical waveguide connected to the optical input end and the optical output end; and a wavelength selector provided on a part of the optical waveguide and reflecting light of a predetermined wavelength. And a sexual reflector,
At least the wavelength-selective reflector provided on the optical output end side of the optical amplifier unit is a blazed grating that returns only a part of the reflected light wave to the optical amplifier and emits the remaining part. Features.
[0012]
Further, one region of the optical waveguide having the blazed grating has at least two or more different propagation modes at least at the predetermined wavelength.
[0013]
The wavelength selective reflector provided at least on the optical output end side of the optical amplifier has a transmittance of 10% or less.
[0014]
Further, the light of the predetermined wavelength output from the optical output terminal of the optical amplifier, the output-side effective reflectance returned to the optical amplifier again through the optical output terminal,
The light of the predetermined wavelength output from the optical input terminal of the optical amplifier is smaller than the input-side effective reflectance returned to the optical amplifier again via the optical input terminal.
[0015]
Preferably, the wavelength-selective reflector is an ultraviolet light-induced grating provided on a silica-based planar optical waveguide.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, specific embodiments will be described in detail with reference to the accompanying drawings.
[0017]
<First embodiment>
FIGS. 1A and 1B are top views showing a gain-clamped optical amplifier according to a first embodiment of the present invention. FIG. 1A is a diagram showing the entirety, and FIG.
[0018]
In the present embodiment, a quartz-based planar optical waveguide 1 formed on a silicon substrate is used, and an SOA element 4 is mounted and fixed on an optical element mounting part 3 formed in a partial region of the optical waveguide 2. By drawing ultraviolet light-induced gratings (hereinafter referred to as UV gratings) 5 and 6 on the optical waveguide 2, a hybrid integrated gain clamp optical amplifier module is formed.
[0019]
The gain clamp SOA can be realized not only by a monolithic integrated configuration integrally formed of semiconductors according to the related art but also by a hybrid integrated configuration as in the present embodiment.
[0020]
In such a hybrid integrated configuration, the coupling portion between the SOA element 4 and the optical waveguide 2 is included in the resonator constituted by the gratings 5 and 6 at both ends. There is an effect that can be suppressed. Therefore, it is advantageous to accurately set the module gain.
[0021]
Note that the structure of the SOA element 4 and the optical element mounting section 3 or the method of fixing the SOA element 4 are not the main points of the present invention, and will not be described in detail. However, reference 2 (I. Ogawa, et al., "Hybrid integrated four-channel SS-SOA array module using planar lightwave circulation circuit 1-system, three-dimensional, three-dimensional, three-dimensional, three-dimensional, three-dimensional, three-dimensional. It goes without saying that many alternatives are possible, including the structure and method described in). Therefore, it should be noted that the structures described in the present specification and drawings are merely examples, and do not particularly limit these structures.
[0022]
Here, a characteristic configuration of the present embodiment will be described. First, in the gain-clamped optical amplifier according to the present embodiment, the output-side UV grating 6 is formed not so that the reflection surface is perpendicular to the light propagation axis of the optical waveguide, but obliquely. did. A so-called "blazed grating" was used. There is no particular limitation on the input side UV grating 5. In this embodiment, a normal UV grating 5 having a reflecting surface formed perpendicular to the optical waveguide 2 is used. In FIG. 1B, 2a is a single mode waveguide, 2b is a tapered portion, and 2c is a multimode waveguide.
[0023]
Usually, in the optical waveguide 2, a grating used as a wavelength selection filter or a wavelength selection mirror is formed so as to be perpendicular to the light propagation axis of the optical waveguide 2. In this case, according to the reflectivity of the grating, a part of the light wave propagating through the optical waveguide 2 and entering the grating is reflected and the remaining part is transmitted, and the component radiated to the outside of the waveguide is Very slight.
[0024]
On the other hand, it is also possible to intentionally form the reflecting surface of the grating obliquely with respect to the optical waveguide axis, such as a “blazed grating”, “tilted grating”, or “slanted grating”. Known by name.
[0025]
As for such a “blazed grating”, there has been reported an example in which a part of a light wave propagating in an optical waveguide is used as an optical output device for extracting the light wave to the outside of the optical waveguide (for example, Reference 3 (Mats Haberg, et al.)). al., "Investigation of high-efficiency surface-emitting lasers with blazed grating outcouplers," IEEE, J.E., pp. 96, 15-n. In the narrow sense, the term “blazed grating” refers only to an optical output coupler that reflects and extracts light above an optical waveguide, or an individual reflection surface is formed obliquely like a sawtooth on a flat substrate. In some cases, it may refer only to the grating for spatial light beam spectroscopy formed in the above, and the definition is not always clear.
[0026]
However, since no other suitable expression is found, the term "blazed grating" is used herein in the following definitions. That is, the “blazed grating” referred to in this specification is a so-called grating having a plurality of reflective surfaces arranged periodically, and the effective reflective surface is not perpendicular to the light incident direction. What is inclined at an angle ".
[0027]
Various structures and manufacturing methods of such a blazed grating have been proposed. In this embodiment, a UV grating 6 drawn on a quartz-based planar optical waveguide 1 is used. In this case, a phase mask is used as an optical waveguide. The blazed grating can be easily formed by, for example, irradiating an ultraviolet ray while being installed obliquely with respect to 2.
[0028]
In each of the drawings attached to this specification, the grating is represented by a plurality of arranged lines, but this is a schematic representation of the state of the reflective surface whose refractive index has been modulated by UV light. However, is not limited to this.
[0029]
For example, in FIG. 1, the lines of the reflection surface extend outside the core of the optical waveguide 2, but these vary depending on the materials of the core and the cladding of the optical waveguide 2, the UV irradiation intensity, the irradiation range, and other conditions. In many cases, it can be considered that the refractive index modulation substantially occurs only inside the core. Also, in FIG. 1, the spacing between the reflecting surfaces and the shading of the color are expressed uniformly over the entire area of the grating. However, the so-called "chirped grating" in which the pitch of the grating reflecting surface is changed along the propagation axis. Alternatively, a so-called “apotized grating” in which the depth of the refractive index modulation is changed for each part of the grating may be used. Such a specific design is not particularly described in the present specification, but it should be understood that it should be appropriately designed in consideration of the reflection spectrum of the grating and the like.
[0030]
Next, a second structural feature of the gain clamp optical amplifier according to the present embodiment is that a region of the optical waveguide 2 provided with the blazed grating is a so-called multi-mode waveguide having at least two or more propagation modes. 2c.
[0031]
As a specific example, in this embodiment, the width of the input / output optical waveguide 2 connected to the SOA element 4 is increased only in the region where the UV grating 6 is formed, thereby enabling excitation of four propagation modes. . The region other than the region where the UV grating 6 is formed is a so-called single-mode waveguide 2a having only one effective propagation mode, and is connected to the multi-mode waveguide 2c by a gentle taper portion 2b.
[0032]
Further, the third and fourth structural features of the gain clamp optical amplifier according to the present embodiment reside in the design of the grating reflectance. That is, the transmittance of each of the UV gratings 5 and 6 on the input side and the output side was set to approximately 0%. Further, the effective reflectivity for the SOA element 4 was designed such that the output side had a sufficiently smaller value than the input side.
[0033]
Here, an outline of the reflectance design in the present embodiment will be described with reference to FIG. 2A and 2B are schematic diagrams showing examples of reflectance design of the input side and output side gratings in the present embodiment, wherein FIG. 2A shows the input side and FIG. 2B shows the output side. As shown in the figure, first, the input-side grating was a normal UV grating 5, and a reflection surface was formed on the single-mode waveguide 2a perpendicular to the waveguide axis. The reflection center wavelength was 1520 nm on the input side, 3 dB reflection bandwidth 0.3 nm, and the reflectance near the center wavelength was 99% or more (transmittance 1% or less).
[0034]
On the other hand, the output side UV grating 6 is the blazed grating described above. Although the reflection spectrum of the UV grating 6 will be described later, the Bragg wavelength determined by the grating pitch viewed along the waveguide axis is set to about 1524 nm so as to oscillate around 1520 nm. In the vicinity of the oscillation wavelength of 1520 nm, the transmittance was 1% or less. The effective reflectance of the blazed grating viewed from the single mode optical waveguide 2a near the SOA element 4 was set to 20%.
[0035]
The effective reflectance (the ratio of the component output from the SOA element 4 and returning to the SOA element 4) as viewed from the SOA element 4 was designed so that the connection loss between the SOA element 4 and the quartz-based planar optical waveguide 1 was about 2 dB. Therefore, the input side is about 63% and the output side is about 13%.
[0036]
With this configuration, according to the present embodiment, a gain-clamped optical amplifier having sufficiently small output intensity of oscillation light and good noise performance can be realized in the present embodiment. This is because the configuration has the following remarkable effects.
[0037]
First, by using the configuration of the UV grating (blazed grating) 6, which is the first and second features, it is possible to design the reflectance and the transmittance independently and with a high degree of freedom. The reason will be described below.
[0038]
In the UV grating (blazed grating) 6, since each reflecting surface is not perpendicular to the incident light but is inclined obliquely by θ from the perpendicular, the reflected wave “basically” has an angle of 2 × θ. It will be reflected in the opposite direction to the incident wave. Since the reflected light has a certain extent in the wavelength axis and the space axis, the power of the light wave reflected in the same direction as the incident wave changes according to the oblique angle θ. That is, considering the case where the incident wave is emitted from the waveguide, only a part of the reflected wave is coupled to the same propagation mode as the incident wave. As described above, in the ordinary UV grating 5 in which the reflection surface is formed perpendicularly to the waveguide axis on the optical waveguide 2, the reflection or the transmission is an alternative. The lower the transmittance, the higher the transmittance. However, this is not the case in the case of a blaze grating. That is, by appropriately setting the oblique angle θ of the reflection surface, the ratio of the reflected light traveling in the direction of the incident light can be appropriately adjusted. On the other hand, the transmittance can be adjusted by the intensity of the grating (such as the degree of refractive index modulation and the length of the grating). Therefore, the effective reflectance and the transmittance can be set effectively independently.
[0039]
Such a design is possible because the phenomenon of "radiating" or "scattering" a part of the reflected wave in a direction different from the desired reflection direction and transmission direction is used. In other words, the effective transmittance and the effective reflectance can be set independently by using the “radiation mode” that is not used in the ordinary waveguide UV grating by using the blazed grating.
[0040]
However, the above-mentioned “basically” means that in the case of the UV grating 6 formed on the quartz-based planar waveguide 1, such a design is difficult even if the grating reflecting surface is simply inclined obliquely. Because it is.
[0041]
For example, in the case of the third embodiment using a dielectric multilayer film to be described later, it is sufficient to consider a grating in a substantially free space, so that the operation substantially as described above can be obtained. However, in contrast, in a waveguide grating formed by slightly modulating the effective refractive index of the waveguide core, the reflected wave is not easily coupled to the radiation mode due to the confinement effect of the waveguide itself. . However, such problems can be avoided by designing a device such as introducing a discontinuous portion in one region of the waveguide confinement structure or providing a high refractive index region very close to the waveguide in the grating formation region. It is possible.
[0042]
A second feature of the present embodiment is that, as an example of such a design, a part of the reflected wave is easily coupled to the radiation mode by widening the waveguide width of the grating forming region to form the multimode waveguide 2c. It was done. This structure can be easily realized by only slightly changing the photomask for forming a waveguide, and as is apparent from the third embodiment described later, for transmitting signal light. Since it is almost lossless, it is extremely useful in practice.
[0043]
FIGS. 3A and 3B are schematic diagrams for explaining the operation of the output side UV grating (blazed grating) 6 in the gain clamp optical amplifier according to the present embodiment, where FIG. 3A shows reflected light, and FIG. 3B shows transmitted light. Is the case. Hereinafter, the operation of the UV grating (blazed grating) 6 formed on the multi-mode waveguide 2c will be described with reference to FIG.
[0044]
The light wave coupled from the SOA element 4 to the quartz-based planar optical waveguide 1 propagates in a single mode, and is then input to the multi-mode waveguide 2c. Here, since the single-mode waveguide 2a and the multi-mode waveguide 2c are connected by a gentle taper portion 2b, even when input to the multi-mode waveguide 2c, no higher-order mode is excited.
[0045]
Next, the light enters the UV grating (blazed grating) 6 and a part of the light wave having a predetermined wavelength is reflected. At this time, the direction of the reflected wave differs depending on the wavelength, and components having wavelengths longer than the Bragg wavelength determined by the pitch of the UV grating 6 along the optical waveguide 2 are reflected almost straight, and most of the components are reflected by the optical waveguide 2. It will return in the opposite direction in the same basic mode as. On the other hand, in a wavelength range slightly shorter than the above wavelength, the reflected wave is reflected at a slight angle with respect to the light propagation axis of the optical waveguide 2, and a component that is coupled to a higher-order mode is generated. These reflected waves return to the multi-mode waveguide 2c and are again input to the single-mode waveguide 2a via the tapered portion 2b, but most of the light reflected in the higher-order mode is transmitted to the single-mode waveguide 2a. It will be radiated into the cladding without coupling to the fundamental mode in the region.
[0046]
Therefore, the UV grating 6 can effectively return only a part of the reflected wave at a specific wavelength and radiate the rest at the connection between the single-mode waveguide 2a and the multi-mode waveguide 2c. On the other hand, the signal light largely deviated from the grating reflection band to the longer wavelength side propagates in the fundamental mode, passes through the UV grating 6, and is connected to the output-side single mode waveguide 2a via the tapered portion 2b. Therefore, the light is transmitted with almost no loss.
[0047]
Such a design can be easily made by using a propagation analysis or the like, and a desired transmittance and an effective reflectance can be obtained by performing calculations using parameters of the optical waveguide 2 and the UV grating 6 as appropriate.
[0048]
4A and 4B are explanatory diagrams for explaining a calculation example of transmission and reflection spectra of the UV grating (blazed grating) 6 of the gain clamp optical amplifier according to the present embodiment. FIG. (b) to (d) are characteristic diagrams at each oblique angle θ. Here, (b), (c), and (d) are characteristic diagrams in the case where the oblique angle θ is 0 °, 1 °, and 2 °, respectively, and all are viewed from the input-side single mode waveguide 2a. 2 shows the calculated results of the reflection spectrum and the transmission spectrum viewed from the output-side single mode waveguide 2a.
[0049]
The intensity of the UV grating 6 is designed so that the reflectance is almost 100%. On the longer wavelength side than the center of the grating reflection band of about 1524 nm, there is no significant change even if the oblique angle θ is changed, and the transmittance increases as the reflectance decreases. On the other hand, near the wavelength of 1521 nm, which is slightly shorter than the above wavelength, the reflectance changes greatly as the oblique angle θ increases, but at this time, the transmittance is maintained at almost 0%.
[0050]
In this calculation example, the reflectance was about 40% when θ = 1 °, and about 10% when θ = 2 °, and the transmittance was able to be maintained at almost 0% in each case. Although only one example is given here, the intensity and the band of the UV grating 6 and the width and the relative refractive index difference Δ of the optical waveguide 2 may be adjusted to obtain a desired reflection and transmission spectrum. Note that, in a wavelength region farther to the shorter wavelength side than the reflection band, the transmittance decreases with the oblique angle θ. This is because a loss is caused by scattering. In such a wavelength range, since the light is not coupled to the propagation mode of the optical waveguide 2, the transmission loss increases while the reflectance remains 0%.
[0051]
On the other hand, as a comparison, a calculation example of transmission and reflection spectra when a UV grating (blazed grating) 6 is similarly formed in the single mode waveguide 2a will be described with reference to FIG. As is clear from the figure, in the UV grating 6 drawn on the single mode waveguide 2a, even if the oblique angle θ of the blazed grating is changed to 0 °, 4 °, 8 °, the change is small, and the reflectance and the transmission It can be seen that the rate cannot be set independently.
[0052]
Note that the design of the optical waveguide 2 as described above is made possible by the effect of using the quartz-based planar optical waveguide 1 in the present embodiment. That is, it is possible to use an optical fiber as the input / output optical waveguide 2 and to use a UV grating formed on the optical fiber as the grating. In that case, however, a special structure is required to obtain the operation as in the present embodiment. Optical fiber processing is required. On the other hand, in the case of the quartz-based planar optical circuit 1, it can be easily realized only by changing the photomask. Further, when compared with a semiconductor optical waveguide, there is an advantage in that such a waveguide design can be manufactured accurately and stably, and in that a UV grating 6 that can easily form a blazed grating can be used.
[0053]
Since the present embodiment has the first and second structural features as described above, the third and fourth features make it possible to appropriately set the reflectance and the transmittance using such a configuration. Thus, a high-performance gain-clamped optical amplifier can be realized.
[0054]
More specifically, first, regarding the second feature, by setting the transmittance of the output side UV grating (blazed grating) 6 to almost 0%, the output intensity of the oscillation light, which has been a problem in the past, can be reduced. It can be kept small. Thus, in many cases, it is not necessary to provide a high-performance filter immediately after the gain clamp SOA module.
[0055]
In this embodiment, since the transmittance is 1% or less on both the input side and the output side, the oscillation light output from either side is slightly 1% or less. Therefore, as shown in the output spectrum of FIG. 6, when the saturation output of the SOA element 4 is 10 dBm, the output power of the oscillating light is about -10 dBm. If it is set to be smaller than the signal light by about 5 to 10 dB, the oscillation light output from the waveguide together with the signal light is suppressed to a power smaller than the signal light by 10 dB or more.
[0056]
Incidentally, in the conventional gain clamp SOA, the grating reflectivity is set to about several% to 20% on both the input side and the output side, and the oscillation light is output at a level about 10 dB larger than the signal light. Such a large-level unnecessary wave causes problems such as saturating the next-stage optical amplifier and inducing non-linearity in the transmission line. Therefore, an optical filter is provided immediately after the gain clamp SOA to provide an oscillation light. Was essential. Further, a high-performance filter having sufficient crosstalk performance is required for the filter used here in order to reduce a large-level oscillation light to a level sufficiently smaller than the signal light.
[0057]
On the other hand, in the gain clamp SOA of the present embodiment, as described above, the oscillation light has a value smaller than the signal light by about 10 dB, so that it is not always necessary to provide a filter immediately after the gain clamp SOA. Even if it is necessary, the oscillation light is originally at a low level, so that an expensive filter with good crosstalk performance is unnecessary.
[0058]
The grating transmittance of the oscillating light is preferably as close to 0% as possible, but is not necessarily limited to this from the viewpoint described above. That is, if the output level of the oscillating light is suppressed to a level that does not cause a problem in the next stage optical amplifier or transmission line, a filter is practically unnecessary, and the same level as the signal light is slightly smaller depending on the application. If so, there is often no problem. For example, since the output intensity of the typical signal light is about 10% of the saturation output, a certain effect can be obtained if the transmittance of the output-side grating is also about 10% or less.
[0059]
Furthermore, in this embodiment, the transmittance of the UV grating 5 on the input side is also set to 1% or less, so that no oscillation light is output from the input side. However, the present invention is not limited to this. When an optical amplifier is used, an isolator is usually provided before the input side and after the output side. Oscillation light output from the input side through the transmission line in the reverse direction is cut off by the isolator. There are many. Therefore, the transmittance and the reflectance on the input side may be determined so as to maximize the characteristics of the gain clamp SOA in consideration of system and circuit conditions.
[0060]
The fourth feature relates to a method of setting the reflectance on the input side and the output side, whereby a gain-clamped optical amplifier with small noise can be obtained. The reason is as follows.
[0061]
In the first place, if the present invention is not used and the output intensity of the oscillating light is merely suppressed, the reflectance of the output side grating may be simply set to almost 100%. That is, if the reflectance of the output-side grating is set to 100%, the oscillation light does not pass through this and is output to the output side. The gain can be adjusted by appropriately setting the reflectance of the input-side grating. In this case, high-level oscillation light is output from the input-side grating in the transmission path in the opposite direction. However, as described above, when an optical amplifier is used, it is necessary to provide an isolator before and after the optical amplifier. Since it is normal, the oscillated light output from the input side of the gain clamp SOA in the transmission path in the reverse direction does not exceed the isolator and does not adversely affect the transmission path.
[0062]
However, such a configuration significantly deteriorates the noise figure NF of the optical amplifier, and is not practically appropriate. Reference 4 (Guido Giuliani, et al., “Noise analysis of conventional and gain-clamped semiconductor product”). amplifiers, "J. of Lightwave Technology, vol. 18, no. 9, pp. 1256-1263, 2000.)), the NF of an optical amplifier depends on the carrier density inside the optical amplifier and its carrier density. There is a problem that the NF is deteriorated depending on the distribution, particularly when saturation occurs on the input end side. In general, it is known that a gain-clamped optical amplifier is used in an oscillating state, so that the NF is deteriorated as compared with a normal SOA. If the ratio is increased, the carrier density decreases on the input end side, so that NF is further deteriorated.
[0063]
On the other hand, if the reflectance on the output side is set sufficiently smaller than the reflectance on the input side as in the present embodiment, the noise on the input side and the output side is lower than that on the normal side. Characteristics can be greatly improved. In the present embodiment, the effective reflectivity for the SOA element is 63% on the input side and 13% on the output side, and the output side is set to be as small as about 1/4 of the input side. NF characteristics were achieved.
[0064]
In this embodiment, the ratio of the effective reflectance between the input side and the output side is set to 4: 1. However, the effect becomes smaller as the output side becomes smaller than 1: 1. For example, a remarkable noise improvement effect can be expected even when compared with a gain clamp optical amplifier having the same ratio at both ends.
[0065]
In summary, the gain-clamped optical amplifier according to the present embodiment is characterized in that (1) the output level of the oscillation light output together with the signal light is extremely small, and (2) the noise is low. These are remarkable effects obtained by the configuration using the UV grating (blazed grating) 6 of the present embodiment and the reflectance design made possible thereby.
[0066]
That is, in the conventional gain clamp optical amplifier, the output power of the oscillating light is large, and it is necessary to provide an externally expensive filter for removal. If the output power of the oscillating light is reduced by increasing the grating reflectivity on the output side to avoid this, there is a problem that NF is degraded, and there is no practical solution.
[0067]
In contrast, in the present embodiment, the use of the UV grating (blazed grating) 6 allows the effective reflectance of the SOA element 4 to be appropriately set while keeping the transmittance of the oscillation light small. By appropriately setting the effective reflectance and transmittance on the input side and output side using this structure, the above-described problems could be solved.
[0068]
In this embodiment, the UV grating 6 formed in the quartz-based planar optical waveguide 1 is used in the hybrid integrated configuration, but a semiconductor monolithic integrated configuration may be used. Multiple configurations are possible. Further, although the configuration using the SOA as the optical amplifier has been described, other materials and structures such as a fiber type optical amplifier may be used.
[0069]
However, the hybrid integrated configuration has the effect of suppressing the module gain fluctuation due to the fluctuation of the coupling loss between the SOA and the different kind of waveguide as described above, and this coupling loss acts only on the oscillation light in a round-trip manner. Since the signal light acts on only a single path, it is advantageous for suppressing the oscillation light. Further, as already described, the quartz-based planar optical waveguide 1 can easily realize a complicated structure in which only the grating region of the present embodiment is a multi-mode waveguide 2c only by the layout of the photomask. This is effective in that such a waveguide can be manufactured with high accuracy and stability. When the UV grating 6 is used, a blazed grating can be easily formed only by inclining the phase mask obliquely, and the grating characteristics such as the reflectance and the bandwidth can be controlled with great flexibility by adjusting the UV irradiation intensity. There are advantages that can be set.
[0070]
In addition, the use of this embodiment has the following effects. That is, in the present embodiment, since the reflectance of the grating itself can be set to approximately 100%, even when the UV grating 6 is used, stable characteristics with little change over time can be obtained. In the UV grating 6, it is known that the refractive index change induced by the UV light slightly changes with time depending on environmental conditions and the like. When the grating reflectance changes over time, the oscillation threshold of the gain-clamped optical amplifier changes, resulting in a problem that the module gain fluctuates. On the other hand, when the reflectance is set to almost 100% as in the present embodiment, if the UV grating 6 is formed in a long area with a sufficient margin in advance, there is a slight change in the refractive index. However, since the reflectance is always 100%, the variation in the reflectance can be suppressed. As described above, when the UV grating 6 is used in combination with the present embodiment, a favorable synergistic effect can be obtained.
[0071]
<Second embodiment>
FIG. 7 is a top view showing a gain-clamped optical amplifier according to the second embodiment of the present invention. As shown in the figure, the difference between the gain clamp optical amplifier according to the present embodiment and the first embodiment is that the optical amplifier 11, the input / output optical waveguides 12, 13, and the gratings 14, 15 are all made of semiconductor. The point is that a monolithic structure is formed integrally. That is, the optical waveguides 12 and 13 formed on both sides of the optical amplification unit 11 are formed as passive optical waveguides, and the upper surfaces of the cores of the optical waveguides 12 and 13 are processed into a relief shape by electron beam exposure, and the gratings 14 and 13 are formed. No. 15 was formed. The grating forming region of the output side optical waveguide 13 was made into a multimode waveguide region 15a by enlarging the core width, and was connected to the surrounding single mode waveguide by a gentle lateral taper. The grating 14 on the input side has a structure having a reflection surface perpendicular to the optical waveguide 12.
[0072]
On the output side, on the other hand, a blazed grating in which the reflecting surface of the grating 15 was inclined with respect to the vertical direction of the optical waveguide 13 was used. The design and operation of the wavelength and the reflectance of the grating 13 are almost the same as those of the first embodiment.
[0073]
However, in the hybrid configuration according to the first embodiment, since optical coupling loss occurs at the connection between the SOA element 4 and the silica-based planar optical waveguide 1 shown in FIG. 1, the reflectance of the UV gratings 5 and 6 is While the connection loss is designed in consideration of the connection loss, in the monolithic structure of the present embodiment, the connection loss between the optical amplification unit 11 and the input / output optical waveguides 12 and 13 shown in FIG. The set values for the rates are slightly different. Specifically, similarly to the first embodiment, the effective reflectances of the input-side and output-side gratings 14 and 15 with respect to the optical amplifier 11 are 63% and 13%, respectively. 37%, so that the oscillation light is output from the grating 12 on the input side. On the other hand, since the grating 13 on the output side was a blazed grating, the transmittance could be set to almost 0% as in the first embodiment.
[0074]
The monolithic structure as in this embodiment is also effective from the viewpoint of miniaturization.
[0075]
In this embodiment, the input and output optical waveguides 12 and 13 are passive optical waveguides, but may be active. That is, the optical amplifier 11 and the input / output optical waveguides 12 and 13 may be collectively formed as an active waveguide. Further, the multi-mode waveguide region 15a has a structure in which the core width is increased in the lateral direction, but may have another structure such as an increase in the thickness direction. Also, the structures of the gratings 14 and 15 need not be relief processing of the core upper surface. For example, relief processing of the core side wall may be performed. Also, it is not necessary to provide the oblique angle in the in-plane direction. For example, as described in Document 3, a grating may be formed diagonally in the vertical direction.
[0076]
<Third embodiment>
FIGS. 8A and 8B are top views showing a gain-clamped optical amplifier according to the third embodiment of the present invention. FIG. 8A is a diagram showing the entirety, and FIG.
[0077]
This embodiment is a hybrid integrated gain-clamped optical amplifier in which the SOA element 24 is mounted and fixed in one region on the silica-based planar optical waveguide 21 as in the first embodiment. The difference from this is that a dielectric multilayer filter 26a that reflects light of a predetermined wavelength and transmits other light is used as the output-side grating (blazed grating) 26. Here, the optical waveguide 22 is a single mode waveguide over the entire area. In such a configuration, a dicing groove 26b having a width of about 50 μm is formed in a part of the optical waveguide 22 on the output side by a dicing saw so as to obliquely cross the optical waveguide 22. It can be realized by inserting 26a and fixing with resin.
[0078]
FIG. 9 is a schematic diagram for explaining the operation of the output-side grating (blazed grating) in the gain-clamped optical amplifier according to the present embodiment, in which (a) is a reflected light and (b) is a transmitted light. is there. As shown in the figure, the light wave emitted from the optical waveguide 22 enters the dielectric multilayer filter 26a via a gap filled with resin. Light waves having a wavelength in the reflection band of the dielectric multilayer filter 26a are reflected, and other light waves are transmitted and coupled to the optical waveguide 22 on the output side of the dielectric multilayer filter 26a. The reflected light returns to the optical waveguide 22 on the SOA element 24 side again. At this time, since an angle shift of approximately 2 × θ occurs, a part of the reflected light is again coupled to the fundamental mode of the optical waveguide 22, but the remaining Will be emitted to the cladding or upper space as a radiation mode. Therefore, the reflectance as viewed from the optical waveguide 22 on the SOA element 24 side can be calculated as an approximate integral of the overlap between the fundamental mode of the optical waveguide 22 and the reflected light, so that the dielectric multilayer filter 26a can obtain a desired reflectance as appropriate. May be designed.
[0079]
Also in this embodiment, the wavelength-selective reflector on the output side is a grating (blazed grating) 26 in which the reflection surface is formed obliquely with respect to the waveguide axis. Thus, the same effect as in the first embodiment can be obtained. That is, by appropriately setting the oblique angle θ, the output intensity of the oscillation light on the output side can be suppressed to almost 0%, and a gain-clamped optical amplifier with good NF can be realized.
[0080]
【The invention's effect】
As described above, the gain clamp optical amplifier of the present invention is characterized in that a blazed grating is used at least as a wavelength-selective reflector on the output side. As a result, the transmittance of the grating and the effective reflectance for the optical amplifier can be controlled independently. A second feature is that only the waveguide region where the blazed grating is provided is a multi-mode waveguide. As a result, even in a grating having a waveguide structure, such as a UV grating on a silica-based optical waveguide, reflected waves can be easily coupled to a radiation mode, and the effective reflectance can be adjusted.
[0081]
In addition to this, by utilizing the effect of the above invention that the transmittance and the effective reflectance can be controlled independently, by designing the following appropriate transmittance and reflectance, the gain-clamped light having desirable properties can be obtained. An amplifier can be realized. That is, by setting at least the transmittance of the output-side grating sufficiently small, it is possible to suppress the output intensity of the oscillating light to be small, and thus it is not necessary to provide an external high-performance filter. If the typical value of the transmittance is 10% or less, a certain effect can be expected.
[0082]
Furthermore, by designing the effective reflectivity for the optical amplifier to be sufficiently smaller on the output side than on the input side, deterioration of NF, which is usually a problem in gain-clamped optical amplifiers, is minimized, and good noise characteristics are obtained. It was decided to be.
In addition, it is preferable to design the effective reflectance as viewed from the optical amplifying unit so that the output side is about 1/4 or less of the input side.
[0083]
Further, it is effective that the blazed grating is a UV grating formed on a quartz-based planar optical waveguide. The reason is that the silica-based optical waveguide is suitable for easily and accurately producing a multi-mode waveguide structure of only a part as described above, and the UV grating can also easily form the blazed grating. Because. Further, since the reflectance design of the present invention is close to 100%, there is also a synergistic effect that the temporal change of the UV grating can be suppressed.
[0084]
Therefore, according to the present invention, it is possible to provide a small, high-performance, and inexpensive gain-clamped optical amplifier having a small output intensity of oscillation light.
[Brief description of the drawings]
FIG. 1 is a top view showing a gain-clamped optical amplifier according to a first embodiment of the present invention, wherein FIG. 1 (a) is an overall view and FIG. 1 (b) is a partially enlarged view.
FIGS. 2A and 2B are schematic diagrams showing examples of reflectance design of an input-side grating and an output-side grating in the first embodiment, where FIG. 2A is an input side and FIG.
FIG. 3 is a schematic diagram for explaining the operation of an output-side UV grating (blazed grating) in the gain-clamped optical amplifier according to the first embodiment, and FIG. ) Is the case of transmitted light.
FIG. 4 is an explanatory diagram for describing an example of calculation of transmission and reflection spectra of a UV grating (blazed grating) of the gain clamp optical amplifier according to the first embodiment, and (a) of FIG. (B) to (d) are characteristic diagrams at each oblique angle θ.
FIG. 5 is an explanatory diagram showing an example of calculation of transmission and reflection spectra when a blazed grating is formed in a single mode waveguide for comparison with FIG. 4;
FIG. 6 is a characteristic diagram showing an example of an output optical spectrum of the gain clamp optical amplifier according to the first embodiment.
FIG. 7 is a top view illustrating a gain-clamped optical amplifier according to a second embodiment of the present invention.
FIGS. 8A and 8B are top views showing a gain-clamped optical amplifier according to a third embodiment of the present invention, wherein FIG. 8A is a diagram showing the entirety, and FIG.
FIG. 9 is a schematic diagram for explaining the operation of an output-side grating (blazed grating) in the gain-clamped optical amplifier according to the third embodiment, where (a) is reflected light and (b) Is the case of transmitted light.
FIG. 10 is a top view schematically showing a configuration example of a conventional gain clamp SOA.
FIG. 11 is a characteristic diagram showing an example of an output light spectrum of a conventional gain clamped SOA.
[Explanation of symbols]
1 Silica-based planar optical waveguide
2 Optical waveguide
2a Single mode waveguide
2c Multimode waveguide
4 SOA element
5, 6 UV grating
11 Optical amplifier
12,13 Optical waveguide
14, 15 grating
15a Multimode waveguide region
21 Silica-based planar optical waveguide
22 Optical waveguide
24 SOA element
25 UV grating
26 grating
26a Dielectric multilayer film filter
26b Dicing groove

Claims (5)

An optical amplifier having an optical input end and an optical output end; an optical waveguide connected to the optical input end and the optical output end; and a wavelength selector provided on a part of the optical waveguide and reflecting light of a predetermined wavelength. And a sexual reflector,
At least the wavelength-selective reflector provided on the optical output end side of the optical amplifier unit is a blazed grating that returns only a part of the reflected light wave to the optical amplifier and emits the remaining part. Features a gain-clamped optical amplifier. The gain-clamped optical amplifier according to claim 1, wherein one region of the optical waveguide including the blazed grating has two or more different propagation modes at least at the predetermined wavelength. 2. The gain-clamped optical amplifier according to claim 1, wherein the transmittance of the wavelength-selective reflector provided at least on the optical output end side of the optical amplifier is 10% or less. The output-side effective reflectivity of the light of the predetermined wavelength output from the optical output end of the optical amplifying unit, which returns to the optical amplifying unit via the optical output end again,
The light of the predetermined wavelength output from an optical input end of the optical amplifying unit is smaller than an input-side effective reflectivity returning to the optical amplifying unit via the optical input end again. 2. A gain clamp optical amplifier according to claim 1. The gain-clamped optical amplifier according to claim 1, wherein the wavelength-selective reflector is an ultraviolet light-induced grating provided on a silica-based planar optical waveguide.

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