JP2019207901A - Optical detection device - Google Patents

Abstract

To improve a sensitivity of an optical detection.SOLUTION: An optical detection device is an optical detection device, comprises: a gain medium for amplifying a detected light to be entered; an optical feedback part that feed-backs a propagation light to be output from the gain medium to the gain medium; and an optical amplification feed-back circuit having the gain medium and the optical feedback part, and the light occurred in the optical amplification feed-back circuit is output from the optical amplification feed-back circuit as an output signal light.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a light detection device.

  With respect to a photodetection device used in the field of optical engineering such as optical communication, optical measurement, and spectroscopy, high-sensitivity detection of the optical power of light to be detected is required. For example, high-sensitivity detection of signal light power and pumping light power in an optical fiber communication system enables high-level understanding of system performance degradation, etc., and advanced control and operation of the system based on the recognition (Non-Patent Documents 1 and 2).

  As a specific example, in a multi-relay transmission system of an optical fiber communication system, fluctuations in signal light power level are detected with an accuracy within about 0.1 dB, and the amount of signal-to-noise ratio degradation at the signal light receiving end is grasped It is necessary to do. As another example, when optically detecting a change in quality of a liquid or food in the spectroscopic field, more detailed and quick quality change information can be obtained by high-sensitivity detection compared to the prior art (Non-patent Document 3). ). As described above, increasing the sensitivity of the optical power detection of the photodetection device is an important basic technology in various applications in the field of optical engineering.

G. P. Agrawal, Fiber-Optic Communication Systems, Third Edition, John Wiley & Sons, Inc. Pp. 250-252, 2002 Yukihiro Ozaki, near infrared spectroscopy, Kodansha, pp. 8-10, 2015 Masuda, Laser Research, 41, 6 pp. 416-422, 2013 E. Desurville, Erbium-Doped Fiber Amplifiers, John Wiley & Sons, Inc. Pp. 524-527, 2002 B. E. A. Saleh, M.M. C. Tich, Fundamentals of Photonics, Second Edition, John Wiley & Sons, Inc. Pp. 1080-1082, 2007

FIG. 22 shows a block diagram of the photodetector 101 in the prior art. When the light to be detected 102 enters the light detection device 101, the light power of the light 102 to be detected is converted into a photocurrent by the light detection element 103. The photocurrent is subjected to signal processing such as amplification at the electric stage in the electric circuit 104 and is extracted as an electric signal corresponding to the optical power of the detected light 102. Here, the optical power of the detected light 102 is optical power _{Pin in} dBm notation, and optical power _Pin0 in mW units. However, the relationship of equation (1) between the optical power _{P in} and the light power _{P in0.}

From the electrical signal taken out by the electric circuit 104, the light power _{P in0} and light power _{P in} is determined, (the error [Delta] P _in) measurement error of the light power _{P in} the value of the optical power _{P in} a certain degree If the signal-to-noise ratio of the light to be detected 2 is sufficiently high, the limit is determined by the performance of the light detection element 103 and the electric circuit 104. An example of the error ΔP _in is 0.1 dB as a typical value of an optical power meter or an optical spectrum analyzer. This means that the relative accuracy of the optical power P _in0 is limited to a constant value regardless of the value of the optical power P _in0 from the relationship of the expression (1). However, when the error of the light power _{P in0} and error [Delta] P _in0, relative error of the light power _{P in0} is given by the relative error ΔP _in0 / _{P in0.} When the value of the error ΔP _in is 0.1 dB, the value of the relative error ΔP _in0 / P _in0 is 0.023, that is, 2.3%.

In the prior art, the operation when the optical amplifier 105 is installed in the previous stage of the light detection element 103 is as follows. Gain Gain in dB representation of the optical amplifier 105 G, the gain of the dimensionless quantity and gain G _0. However, in general, an optical amplifier is used under a constant gain or almost constant operating condition. The output light power of the detected light 102 at the output stage of the optical amplifier 105 is set as output light power P _out in dBm notation and output light power P _out0 in mW units. In this case, there is a relationship of P _out0 = GP _in0 . Here, in this relational expression, P _out0 represents the output optical power P _out0 , and P _in0 represents the input optical power P _in0 . In this relational expression, G is a proportionality constant. Further, _assuming that the error of the output optical power is the error ΔP _out0 , there is a relationship of ΔP _out0 = GΔP _in0 . In this relational expression, ΔP _out0 represents an error ΔP _out0 , and ΔP _in0 represents an error ΔP _in0 . Therefore, the relative error ΔP _out0 / P _out0 of the output optical power is expressed by Equation (2).

According to equation (2), the relative error ΔP _out0 / P _out0 is equal to the relative error of the input optical power. That is, when the optical amplifier 105 is used, the relative accuracy in the optical power measurement of the detected light 102 is constant and does not improve. That is, in the photodetection device 101, the sensitivity of photodetection is constant and does not improve.
With respect to the amount expressed in dB, there is a relationship of Equation (3).

  The optical amplifier 105 can be used to improve the signal-to-noise ratio when the optical power of the input signal light is weak and the signal-to-noise ratio of the input signal light is small. (Non-Patent Documents 1, 3, 4, 5).

  The present invention has been made in view of the above points, and provides a light detection device capable of improving the sensitivity of light detection.

  The present invention has been made to solve the above problems, and one aspect of the present invention provides a gain medium that amplifies input light to be detected, and propagation light output from the gain medium. And a light amplification feedback circuit including the gain medium and the light feedback unit, wherein the light generated in the light amplification feedback circuit is transmitted from the light amplification feedback circuit. It is a light detection device that outputs as output signal light.

  In one embodiment of the present invention, in the above-described photodetector, the optical feedback unit includes a narrow-band light transmission filter.

  In one embodiment of the present invention, in the above-described light detection device, the detected light is excitation light of the gain medium.

  In one embodiment of the present invention, in the above-described photodetector, the detected light is signal light of the gain medium.

  In one embodiment of the present invention, in the above photodetector, the optical feedback unit includes an optical fiber as an optical waveguide.

  In one embodiment of the present invention, in the above-described photodetector, the optical feedback unit has a free space as an optical waveguide.

  In one embodiment of the present invention, in the above-described light detection device, the optical feedback portion has a ring shape.

  In one embodiment of the present invention, in the above-described light detection device, the optical feedback unit has a linear shape.

  In one embodiment of the present invention, in the above-described optical detection device, the optical feedback unit includes an optical circulator, and the input of the detected light and the output of the output signal light via the optical circulator. I do.

  In one embodiment of the present invention, in the above-described photodetector, the optical amplification feedback circuit includes a bias light source for the detected light.

  One embodiment of the present invention is the above-described photodetector, wherein the gain medium is a rare earth-doped fiber.

  One embodiment of the present invention is the above-described photodetector, wherein the gain medium is a laser diode.

  According to another aspect of the present invention, in the above-described photodetector, an optical amplifier for amplifying the detected light is provided in a stage preceding the optical amplification feedback circuit.

  According to another aspect of the present invention, in the above-described photodetector, an optical amplifier for amplifying the output signal light is provided after the optical amplification feedback circuit.

  According to the present invention, the sensitivity of light detection can be improved.

It is a block diagram which shows an example of the basic composition of the photon detection apparatus concerning this invention. It is a block diagram which shows an example of the optical amplification feedback circuit of this invention. It is a figure which shows an example of operation | movement of the optical amplification feedback circuit of this invention. It is a block diagram which shows an example of the optical amplification feedback circuit in the photon detection apparatus which concerns on the 1st Embodiment of this invention. It is a block diagram which shows an example of the optical amplification feedback circuit in case the gain medium which concerns on the 1st Embodiment of this invention is an erbium addition fiber. It is a block diagram which shows an example of the whole structure of the photon detection apparatus which concerns on the 1st Embodiment of this invention. It is a figure which shows an example of the calculation experiment result of the optical input-output characteristic which concerns on the 1st Embodiment of this invention. It is a figure which shows an example of the plot of the inclination with respect to the excitation light power which concerns on the 1st Embodiment of this invention. It is a block diagram which shows an example of the optical amplification feedback circuit which concerns on the 2nd Embodiment of this invention. It is a figure which shows an example of operation | movement of the optical amplification feedback circuit which concerns on the 2nd Embodiment of this invention. It is a block diagram which shows an example of the optical amplification feedback circuit in case the gain medium based on the 2nd Embodiment of this invention is an erbium addition fiber. It is a block diagram which shows an example of the optical amplification feedback circuit in case the gain medium which concerns on the 2nd Embodiment of this invention is a laser diode. FIG. 6 is a block diagram showing an example of an optical amplification feedback circuit when a gain medium according to a second embodiment of the present invention is a laser diode and an optical waveguide is a free space. It is a block diagram which shows an example of the optical amplification feedback circuit in the photodetector which concerns on the 3rd Embodiment of this invention. It is a block diagram which shows an example of the optical amplification feedback circuit in the photodetector which concerns on the 4th Embodiment of this invention. It is a block diagram which shows an example of the optical amplification feedback circuit in the photodetector which concerns on the 5th Embodiment of this invention. It is a block diagram which shows an example of the optical amplification feedback circuit in case the gain medium which concerns on the 5th Embodiment of this invention is a laser diode, and the optical waveguide 21 is free space. It is a block diagram which shows an example of the optical amplification feedback circuit in the photodetector which concerns on the 6th Embodiment of this invention. It is a block diagram which shows an example of the optical amplification feedback circuit in case the gain medium based on the 6th Embodiment of this invention is an erbium addition fiber. It is a block diagram which shows an example of the optical amplification feedback circuit of the photon detection apparatus which concerns on the 7th Embodiment of this invention. It is a block diagram which shows an example of the optical amplification feedback circuit which concerns on the 7th Embodiment of this invention. It is a block diagram which shows an example of the photon detection apparatus in a prior art.

The light detection device of the present invention has the following features and operations as main features.
FIG. 1 shows a block diagram of a basic configuration of a light detection apparatus 1 according to the present invention. The light detection device 1 in FIG. 1 is different from the conventional light detection device 101 in FIG. 22 in the following points. That is, in the light detection device 1, the optical amplification feedback circuit 6 is installed in the previous stage of the light detection element 3. The light to be detected 2 enters the light detection element 3 via the optical amplification feedback circuit 6.

  A block diagram of the optical amplification feedback circuit 6 is shown in FIG. The optical amplification feedback circuit 6 includes a gain medium 11, an optical feedback unit 12, an optical input unit 13, and an optical output unit 14. Here, the gain medium is an optical amplification medium. Feedback is feedback.

  The detected light 2 enters the optical amplification feedback circuit 6 from the optical input unit 13 and enters the gain medium 11. Here, the wavelength of the light to be detected 2 matches the excitation light wavelength in the optical excitation of the gain medium 11. The gain medium 11 excited by the detected light 2 outputs amplified spontaneous emission (ASE) light as propagating light in the optical waveguide. That is, the gain medium 11 amplifies the input detected light 2.

The ASE light output from the gain medium 11 enters the gain medium 11 again via the optical feedback unit 12. Therefore, the optical feedback unit 12 feeds back the propagation light output from the gain medium 11 to the gain medium 11.
The optical amplification feedback circuit 6 operates at an operating point below the vicinity of the laser oscillation threshold. The ASE light generated in the optical amplification feedback circuit 6 is output from the optical output unit 14 as signal light having optical power information of the detected light 2. The output light output from the light output unit 14 is referred to as output signal light 7. Therefore, the light detection apparatus 1 outputs the light generated in the optical amplification feedback circuit 6 as the output signal light 7 from the optical amplification feedback circuit 6.

The operation of the optical amplification feedback circuit 6 having the basic configuration shown in FIG. 2 will be described below with reference to FIG. A plot PL3 shown in FIG. 3 shows input / output characteristics of the optical amplification feedback circuit 6. The input light is detected light 2 and acts as excitation light for the gain medium 11. That is, the detected light 2 is excitation light for the gain medium 11. The output light is the output signal light 7 output from the light output unit 14.
The optical amplification feedback circuit 6 operates _in the range of the detected light power below the vicinity of the laser oscillation threshold, that is, the range of the optical power Pin of the detected light 2. The amount of change in the optical power _{P in} the amount of change ΔP _in. Also, when the amount of change in the optical power _{P in} delta increases, the amount of change [Delta] P _out the amount of change in the optical power _{P out} of the output signal light 7.

  As a result of examining the operation of the optical amplification feedback circuit 6, it was found that Equation (4) is obtained.

  Therefore, with respect to the performance of the optical amplification feedback circuit 6 shown in FIG.

  This slope S can be considered to represent the optical power detection sensitivity. Therefore, in the present specification, this slope S is appropriately referred to as optical power detection sensitivity. In the prior art, S = 1 from equation (3). Therefore, by using the optical amplification feedback circuit 6 of the present invention, the slope is S times that of the prior art, that is, the optical power detection sensitivity is S times. As described above, since the optical power detection error of the photodetecting element 3 and the circuit subsequent to the photodetecting element 3 is limited, the optical power detection sensitivity of the detected light 2 is increased by S times according to the present invention. be able to.

In particular, by installing the narrow-band light transmission filter OBPF in the optical feedback unit 12, the slope S can be set to a value sufficiently larger than 1, for example, 10 times or 100 times. Incidentally, the equation (5), the minute change amount expressed in mW units [Delta] P _in0, and is represented by small amount of change [Delta] P _out0, the equation (6).

The right side second factor ΔP _out0 / P _{in0 in} equation (6) is larger as the pumping light power P _in0 is larger, but the right side first factor P _in0 / P _out0 is smaller as the pumping light power P _in0 is larger. Have.

  As described above, the photodetector 1 according to the present invention includes the gain medium 11, the optical feedback unit 12, and the optical amplification feedback circuit 6. The gain medium 11 amplifies the input detected light 2. The optical feedback unit 12 feeds back the propagation light output from the gain medium 11 to the gain medium 11. The optical amplification feedback circuit 6 includes a gain medium 11 and an optical feedback unit 12. The light detection device 1 outputs the light generated in the optical amplification feedback circuit 6 as the output signal light 7 from the optical amplification feedback circuit 6.

With this configuration, the photodetector 1 according to the present embodiment, an optical amplifier feedback circuit 6 a threshold proximity following the detected light power of the laser oscillation, i.e., be operated in a range of the optical power P _in of the light to be detected 2 Therefore, the sensitivity of light detection can be improved.

Further, in the light detection device 1 according to the present invention, the detected light 2 is excitation light of the gain medium 11.
With this configuration, in the light detection device 1 according to the present invention, there is no need to install an excitation light source that outputs excitation light in the light detection device 1, and the device can be simplified.

(First embodiment)
FIG. 4 is a block diagram showing the optical amplification feedback circuit 6a in the photodetector 1 according to the first embodiment of the present invention. The basic configuration of the photodetecting device 1 of the first embodiment is shown in FIG. In FIG. 4, the gain medium 11 is connected in a ring shape using an optical waveguide 21. That is, in the optical ring circuit ORC configured using the optical waveguide 21, light emitted from the gain medium 11 is transmitted as in-circuit propagation light 22 (in-circuit propagation light 22-1 to in-circuit propagation light 22-4). The light propagates in the direction of the arrow shown in FIG. The optical ring circuit ORC is an example of the optical feedback unit 12.

  In the optical ring circuit ORC, the optical isolator ISO installed after the gain medium 11 restricts the propagation direction of the in-circuit propagation light 22 in the optical ring circuit ORC in the clockwise direction. However, counterclockwise propagating light may be generated without installing the optical isolator ISO. The optical isolator ISO also has a function of suppressing the residual reflected light with respect to the gain medium 11 generated in the photodetector 1 and stabilizing the operation of the optical amplification feedback circuit 6a.

A narrow-band light transmission filter OBPF is installed as appropriate after the optical isolator ISO. Therefore, the optical ring circuit ORC, that is, the optical feedback unit 12 includes the narrowband light transmission filter OBPF.
The branching device BR installed at the subsequent stage of the narrow-band light transmission filter OBPF branches a part of the intra-circuit propagation light 22 outside the optical ring circuit ORC. The light branched by the branching device BR is output from the optical amplification feedback circuit 6 a as the output signal light 7 through the optical output unit 14.

  The optical attenuator ATT installed at the subsequent stage of the branching device BR sets the optical loss L of the optical ring circuit ORC to an appropriate value. A multiplexer CP-P is installed after the optical attenuator ATT. The multiplexer CP-P introduces the detected light 2 input to the optical amplification feedback circuit 6a via the optical input unit 13 into the optical ring circuit. Here, the optical loss L is an optical loss from the output unit to the input unit of the gain medium 11.

FIG. 5 shows a block diagram of the optical amplification feedback circuit 6b when the gain medium 11 is an erbium-doped fiber EDF in the first embodiment of the present invention shown in FIG. Regarding the optical amplification feedback circuit 6b of FIG. 5, main points different from the optical amplification feedback circuit 6a of FIG. 4 are described below.
The erbium-doped fiber EDF is a rare earth-doped fiber. Therefore, in the optical amplification feedback circuit 6b, the gain medium 11 is a rare earth doped fiber.

  In the optical amplification feedback circuit 6 b of FIG. 5, an optical fiber OF is used as the optical waveguide 21. Here, the optical ring circuit ORCb included in the optical amplification feedback circuit 6 b, that is, the optical feedback unit 12 includes an optical fiber OF as the optical waveguide 21. In the optical amplification feedback circuit 6b, the optical connector 13b and the optical connector 14b are used as the optical input unit 13 and the optical output unit 14, respectively. The optical fiber OF is a G.G. A polarization non-maintaining type such as 652 or a polarization maintaining type such as PANDA fiber is used.

  In the optical amplification feedback circuit 6b, the optical isolator at the rear stage of the gain medium 11, that is, the erbium-doped fiber EDF is the optical isolator ISO-B. Further, in the optical amplification feedback circuit 6b, an optical isolator ISO-F is installed before the erbium-doped fiber EDF so as to suppress the residual reflected light to the erbium-doped fiber EDF.

In the optical amplification feedback circuit 6b, an optical fiber coupler is used as the branching device BR. The branching ratio of the branching device BR is 90:10 as an example. However, the branching ratio 90 is the ratio in the optical ring circuit ORCb. Another example of the branching ratio of the branching device BR is 50:50 or the like. In the optical amplification feedback circuit 6b, for example, the wavelength of the detected light 2 is set to 1480 nm, which is one of the wavelengths of the excitation light of the erbium-doped fiber EDF. Another example of the excitation light wavelength is 980 nm. As an example, the wavelength of the output signal light 7 is 1558 nm, which is one of the signal light wavelengths of the erbium-doped fiber EDF. Other examples of the signal light wavelength include 1530 to 1610 nm as the signal light wavelength of the erbium-doped fiber amplifier EDFA.
In the optical amplification feedback circuit 6b, the optical fiber OF is used as the optical waveguide 21, but a configuration using a lens, a free space, and a mirror may be used as the optical waveguide 21.

In FIG. 6, the block diagram of an example of the whole structure of the photon detection apparatus 1 regarding 1st Embodiment of this invention is shown. The detected light 2 transmitted from the light source OS in FIG. 6 is input to the core C1 of the multi-core fiber MCF as an optical waveguide.
The detected light 2 that has propagated through the core C1 reaches the first system S1 for light detection, and a part of the light is branched by the branching device B1 and guided to the optical amplification feedback circuit 6. The remaining detected light 2 branched in the branching device B1 is guided to another system (after the system S2 (not shown)) installed downstream of the core C1.

  A measurement object DUT is installed between the branching device B1 and the optical amplification feedback circuit 6. The measurement object DUT is, for example, an optical component as a loss medium, a liquid, a gas, an individual as a spectroscopic measurement object, an optical amplifier as a gain medium, or the like. That is, the photodetection device 1 detects loss and gain fluctuation of the DUT to be measured with high sensitivity. The present invention can also be carried out when there is no DUT to be measured. In this case, the light detection device 1 detects the light power fluctuation of the light 2 to be detected from the light source OS with high sensitivity.

FIG. 7 shows an example of a calculation experiment result of the optical input / output characteristics in the first embodiment of the present invention shown in FIG. The horizontal axis shows the pumping light power of the EDF input. Here, the pumping light power of the EDF input is the optical power P _in the detected light 2 as defined in dBm notation. The vertical axis represents the signal light power of the EDF output. Here, the signal light power of the EDF output is the light power of the output signal light 7 defined in dBm notation. However, the flat gain in the C band of the erbium-doped fiber EDF was about 20 dB, and the optical loss L was 15 dB.
FIG. 7 shows plots in order from the top when there is no narrowband light transmission filter OBPF and when the transmission bandwidth BW of the narrowband light transmission filter OBPF is 10 nm, 1 nm, 0.1 nm, and 0.01 nm. .

Also, the slope S given in equation (5) described above, the value of the case small change amount [Delta] P _in the P _in is sufficiently small, as shown in FIG. In FIG. 8, when there is no narrowband light transmission filter OBPF and when the transmission bandwidth BW of the narrowband light transmission filter OBPF is 10 nm, 1 nm, 0.1 nm, and 0.01 nm, The plots are shown in order from the bottom.

FIG. 8 reveals that the slope S increases as the transmission bandwidth BW decreases. When the maximum value of the slope S is obtained, the case where there is no narrowband light transmission filter OBPF and the case where the transmission bandwidth BW of the narrowband light transmission filter OBPF is 10 nm, 1 nm, 0.1 nm, and 0.01 nm are sequentially obtained. 10.8, 19.3, 57.5, 178.6, 555.6. Further, the pumping light power _{P in} providing their maximum value, when there is no narrow-band light transmitting filter OBPF, and the transmission band width BW of the narrow-band light transmitting filter OBPF is, 10 nm, 1 nm, 0.1 nm, 0. In the case of 01 nm, they were 7.93 dBm, 8.02 dBm, 8.07 dBm, 8.07 dBm, and 8.08 dBm, respectively. The output signal light power P _out is in the order of the case where there is no narrowband light transmission filter OBPF and the case where the transmission bandwidth BW of the narrowband light transmission filter OBPF is 10 nm, 1 nm, 0.1 nm, 0.01 nm. They were about -7.0 dBm, about -9.5 dBm, about -14.0 dBm, about -19.5 dBm, and about -23.5 dBm.

  Setting the transmission bandwidth BW to about 1 nm to 0.1 nm can be realized with an easily available optical component. Therefore, according to the present embodiment, the slope S, that is, the photodetection sensitivity can be increased to about 10 to 100 times, and a remarkable increase in photodetection sensitivity can be achieved as compared with the prior art.

As described above, in the light detection device 1 according to the present invention, the optical feedback unit 12 includes the narrow-band light transmission filter OBPF.
With this configuration, in the light detection device 1 according to the present invention, the light detection sensitivity can be significantly improved as compared with the case where the narrow band light transmission filter OBPF is not provided.

In the light detection device 1 according to the present invention, the optical feedback unit 12 includes the optical fiber OF as the optical waveguide 21.
With this configuration, in the optical detection device 1 according to the present invention, the optical waveguide 21 is constructed using the optical fiber OF, so that the optical detection device 1 is not constructed using the optical fiber OF. Can be stabilized.

In the light detection device 1 according to the present invention, the gain medium 11 is a rare earth-doped fiber.
With this configuration, in the photodetector 1 according to the present invention, the gain medium 11 can be stabilized as compared with a case where the gain medium 11 is not a rare earth-doped fiber by using the rare-earth doped fiber.

(Second Embodiment)
FIG. 9 is a block diagram showing an optical amplification feedback circuit 6c in the photodetector 1 according to the second embodiment of the present invention. The optical amplification feedback circuit 6c of FIG. 9 and the optical amplification feedback circuit 6a of the first embodiment of FIG. 4 are mainly different in the following points.

In the second embodiment, an excitation source 15 for the gain medium 11 is installed. The excitation source 15 excites the gain medium 11 to generate gain. The type of the excitation source 15 varies depending on the type of the gain medium 11. For example, when the gain medium 11 is an erbium-doped fiber EDF, the excitation light source 15d. When the gain medium 11 is a laser diode LD, it is a current source 15e that supplies a drive current for the laser diode LD.
The detected light 2 is light having a wavelength within the gain band wavelength region of the gain medium 11. A multiplexer CP-A is provided between the optical input unit 13 and the gain medium 11 to multiplex the detected light 2 and the intra-circuit propagation light 22 in the optical ring circuit ORCc. The multiplexer CP-A is, for example, a dielectric multilayer filter type multiplexer.

  The operation and action of the optical amplification feedback circuit 6c of the second embodiment will be described with reference to FIG. The operation of the optical amplification feedback circuit 6a of FIG. 3 for the first embodiment is an operation for the case where the detected light 2 is the excitation light of the gain medium 11, and the increase of the excitation light power is the inversion distribution amount of the gain medium 11. The optical power of the output signal light 7 increases with the pumping light power. On the other hand, in the present embodiment, the detected light 2 is a signal light of the gain medium 11, and an increase in the power of the signal light causes a decrease in the inversion distribution amount of the gain medium 11.

Therefore, as shown in FIG. 10, by plotting the plot PL10 of the detected optical power so as to increase from right to left on the paper surface of the horizontal axis, the plot PL10 of FIG. The curve shape is similar to that of the plot PL3. Here, the plot PL10 and the plot PL3 have the same curve shape means that the plot PL10 and the plot PL3 can be overlapped. The amount of change in the optical power _{P in} the amount of change ΔP _in. Also, when the change amount [Delta] P _in the optical power _{P in} is reduced, and the variation [Delta] P _out the amount of change in the optical power _{P out} of the output signal light 7. As a result, an operation similar to the operation described in the equations (4) and (5) is established.

FIG. 11 is a block diagram when the gain medium 11 according to the second embodiment is an erbium-doped fiber EDF. The optical amplification feedback circuit 6d in FIG. 11 is similar to the optical amplification feedback circuit 6b in FIG. 5, but the main differences are described below.
The optical amplification feedback circuit 6d includes an excitation light source 15d of an erbium-doped fiber EDF as an excitation source. In the optical amplification feedback circuit 6d, the case where the wavelength of the excitation light source 15d is 1480 nm is shown. In the optical amplification feedback circuit 6d, as the detected light 2, light in the EDF gain wavelength region is used as input signal light. In the optical amplification feedback circuit 6d, the wavelength of the input signal light is 1550 nm.

FIG. 12 is a block diagram showing an optical amplification feedback circuit 6e when the gain medium 11 in the second embodiment is a laser diode LD. The optical amplification feedback circuit 6e of FIG. 12 includes a current source 15e that supplies a drive current for the laser diode LD as an excitation source.
As an example, the wavelength of the light to be detected 2 is 1530 nm to 1550 nm. Moreover, the wavelength of the output signal light 7 in this case is 1560 nm or 1520 nm as an example. However, the gain wavelength region of the laser diode LD covers the above-described wavelength, and is, for example, 1510 nm to 1570 nm. The gain wavelength range of the laser diode LD is known to cover a wide wavelength range by changing the composition of the laser diode LD, and covers the wavelength range from visible light to near infrared, for example.

FIG. 13 is a block diagram showing an optical amplification feedback circuit 6f when the gain medium 11 is a laser diode LD and the optical waveguide 21 is a free space in the second embodiment. Since the optical amplification feedback circuit 6f in FIG. 13 is similar to the optical amplification feedback circuit 6e in FIG. 12, the main differences from the optical amplification feedback circuit 6e in FIG. 12 will be described below.
In the optical amplification feedback circuit 6e of FIG. 12, an optical fiber is used as the optical waveguide 21, but in the optical amplification feedback circuit 6f of FIG. 13, the optical waveguide 21 includes free space and four mirrors M (mirrors M1 to M4). It is comprised using. Here, the optical ring circuit ORCf, that is, the optical feedback unit 12 has a free space as the optical waveguide 21.

  The optical waveguide 21 has a topological ring shape. Therefore, the optical ring circuit ORCf, that is, the optical feedback unit 12 has a ring shape. The optical waveguide 21 has a topological ring shape, but the number of mirrors M is not limited to the number in the case of FIG. 13 and may be three or more.

As described above, in the photodetector 1 according to the present invention, the detected light 2 is the signal light of the gain medium 11.
With this configuration, in the light detection device 1 according to the present invention, an increase in the power of the signal light causes a decrease in the amount of inversion distribution of the gain medium 11, so that the sensitivity of light detection can be improved.

In the light detection device 1 according to the present invention, the gain medium 11 is a laser diode LD.
With this configuration, in the photodetector 1 according to the present invention, the gain wavelength region of the laser diode LD can cover the wavelength of the detected light 2 and the wavelength of the output signal light 7, so that the bandwidth of the photodetector 1 can be increased. be able to.

In the light detection device 1 according to the present invention, the optical feedback unit 12 has a free space as the optical waveguide 21.
With this configuration, in the light detection device 1 according to the present invention, the polarization state of light propagating through the optical waveguide 21 is maintained, so that the light detection device 1 does not have a free space as the optical waveguide 21. Can be stabilized.

Moreover, in the photodetection device 1 according to the present invention, the optical feedback unit 12 has a ring shape.
With this configuration, in the light detection device 1 according to the present invention, the propagation light becomes a single transmission with respect to the optical component installed in the optical feedback unit 12, so that the requirements for the characteristics of the optical component are relaxed. Can do.

(Third embodiment)
FIG. 14 is a block diagram showing an optical amplification feedback circuit 6g in the photodetector 1 according to the third embodiment of the present invention. The optical amplification feedback circuit 6g shown in FIG. 14 is similar to the block diagram showing the optical amplification feedback circuit 6c of the second embodiment shown in FIG. 9, and therefore, the differences from the optical amplification feedback circuit 6c shown in FIG. Shown in
In the optical amplification feedback circuit 6g, the optical waveguide 21 has a linear topology arrangement, that is, a linear shape, and both ends of the optical waveguide 21 are two mirrors, one at each end (a mirror M-1 and a mirror). M-2) is installed. Therefore, the optical ring circuit ORCg, that is, the optical feedback unit 12 has a linear shape.

  The optical waveguide 21 is an optical fiber or free space, and the in-circuit propagation light 22 reciprocates in the optical waveguide 21. As a result, the intra-circuit propagation light 22 passes through the gain medium 11 twice while the intra-circuit propagation light 22 reciprocates once in the optical waveguide 21. Similarly, the in-circuit propagation light 22 passes twice through the optical components such as the narrow-band light transmission filter OBPF and the optical attenuator ATT. The second embodiment and the third embodiment described above have the above-described differences, but there is no essential difference regarding the characteristics shown in FIGS. 3 and 10. Therefore, as shown in the second embodiment, the third embodiment can improve the optical power detection sensitivity.

As described above, in the light detection device 1 according to the present invention, the optical feedback unit 12 has a linear shape.
With this configuration, in the light detection device 1 according to the present invention, the number of components can be reduced as compared with the case of the ring shape.

(Fourth embodiment)
FIG. 15 is a block diagram showing an optical amplification feedback circuit 6h in the photodetector 1 according to the fourth embodiment of the present invention. Since the optical amplification feedback circuit 6h in FIG. 15 is similar to the optical amplification feedback circuit 6g of the third embodiment in FIG. 14, the following mainly describes changes from the optical amplification feedback circuit 6g in FIG.
In FIG. 15, partial reflection mirrors (mirror M-1 and mirror M-2) are installed at both ends of the gain medium 11. The intra-circuit propagation light 22 receives feedback between the mirror M-1 and the mirror M-2. The detected light 2 passes through the light input unit 13 and the mirror M-1 and enters the gain medium 11. The intra-circuit propagation light 22 propagated from the gain medium 11 in the direction of the mirror M-2 passes through the optical isolator ISO, and then is output from the optical amplification feedback circuit 6h as the output signal light 7 through the optical output unit 14. As an example, the gain medium 11 is a laser diode LD having cleaved surfaces at both ends. When the laser diode LD is a DFB type or DBR type, the gain medium 11 has a function of a narrow band light transmission filter OBPF.

(Fifth embodiment)
A block diagram showing an optical amplification feedback circuit 6i in the photodetector 1 of the fifth embodiment of the present invention is shown in FIG. Since the optical amplification feedback circuit 6i of FIG. 16 is similar to the optical amplification feedback circuit 6h of the fourth embodiment of FIG. 15, the following mainly describes changes from the optical amplification feedback circuit 6h of FIG.

In the optical amplification feedback circuit 6 i, an optical circulator 17 is installed between the optical input unit 13 and the gain medium 11. On the other hand, the optical isolator ISO is not installed after the mirror M-2. The detected light 2 enters the gain medium 11 after passing through the optical circulator 17 and the mirror M-1. The detected light 2 emitted from the gain medium 11 is reflected by the mirror M-2 and propagates in the opposite direction. Thereafter, the in-circuit propagating light 22 that is incident on the gain medium 11, propagates through the gain medium 11, exits from the gain medium 11, and passes through the mirror M- 1 is transmitted from the right port PR 1 of the optical circulator 17 to the lower port. The light is guided to PD 1 and becomes output signal light 7.
Therefore, the optical ring circuit ORCi, that is, the optical feedback unit 12 includes the optical circulator 17, and inputs the detected light 2 and outputs the output signal light 7 through the optical circulator 17.

  In FIG. 16, the case where the gain medium 11 is a laser diode LD and the optical waveguide 21 is a free space is shown in FIG. In the optical amplification feedback circuit 6j of FIG. 17, the optical input unit 13 and the optical output unit 14 are optical connectors, and a lens LS (lens LS1 and LS2) is installed on the right side thereof. In the optical amplification feedback circuit 6j, a lens LS3 is also provided between the laser diode LD and the optical circulator 17. The propagation light between the lens LS1 and the lens LS3 and the propagation light between the lens LS3 and the lens LS2 are collimated beams. The mirror M disposed between the optical circulator 17 and the lens LS2 has a function of changing the propagation direction of the collimated beam.

As described above, in the optical detection device 1 according to the present invention, the optical feedback unit 12 includes the optical circulator 17, and the input of the detected light 2 and the output signal light 7 through the optical circulator 17. Output.
With this configuration, in the light detection device 1 according to the present invention, the output signal light 7 is extracted more efficiently than when the detected light 2 and the output signal light 7 are not input via the optical circulator 17. Can do.

(Sixth embodiment)
FIG. 18 is a block diagram showing an optical amplification feedback circuit 6k in the photodetector 1 according to the sixth embodiment of the present invention. The optical amplification feedback circuit 6k of FIG. 18 is similar to the optical amplification feedback circuit 6a of the first embodiment of FIG. 4, and therefore, changes from the optical amplification feedback circuit 6a of FIG. 4 are mainly shown.

  In the optical amplification feedback circuit 6k, a bias light source 18 for the detected light 2 is installed. Further, a multiplexer CP-B for multiplexing the detected light 2 and the bias light from the bias light source is installed between the optical input unit 13 and the multiplexer CP-A. The detected light 2 and the bias light pass through the multiplexer CP-B and the multiplexer CP-A and enter the gain medium 11. The detected light 2 and the bias light both have close wavelengths and function as excitation light of the gain medium 11 or signal light amplified by the gain medium 11.

In the optical amplification feedback circuit 6a of the first embodiment shown in FIG. 4, the optical power level of the detected light 2 needs to be a certain level. Here, the value of the magnitude is called a required level. The required level is, for example, several dBm. Therefore, in the detected light 2 having a light power smaller than the required level, the light detection sensitivity may be deteriorated.
On the other hand, in the optical amplification feedback circuit 6k of the sixth embodiment in FIG. 18, the optical power obtained by combining the optical power of the detected light 2 and the optical power of the bias light only needs to reach a required level. Therefore, the optical amplification feedback circuit 6k of this embodiment has an advantage that the optical power of the detected light 2 may be smaller than that of the optical amplification feedback circuit 6a of the first embodiment.

FIG. 19 is a block diagram when the gain medium 11 is an erbium-doped fiber EDF in FIG. Since the optical amplification feedback circuit 6l in FIG. 9 is similar to the optical amplification feedback circuit 6d in the second embodiment in FIG. 11, the following mainly describes changes from the optical amplification feedback circuit 6d in FIG.
In the optical amplification feedback circuit 61, a light source that transmits signal light to the erbium-doped fiber EDF is installed as the bias light source 18. The wavelength of the signal light transmitted from the bias light source 18 with respect to the wavelength 1550 nm of the input signal light that is the detected light 2 was set to the near wavelength of 1540 nm. As an example, the multiplexer CP-B is a dielectric multilayer filter that multiplexes signal light of 1550 nm and bias light of 1540 nm.

In the embodiment of FIG. 19, the required level of optical power is 6 dBm (ie, 4 mW) as an example. In this case, the optical power of the detected light 2 is 0.01 mW, that is, −20 dBm. In this case, the optical power of the bias light is 3.99 mW, that is, about 6 dBm.
Thus, the optical power of the detected light 2 is 6 dBm in the optical amplification feedback circuit 6d of the second embodiment of FIG. 11, whereas in the optical amplification feedback circuit 6l of the sixth embodiment of FIG. The level is -20 dBm. In the optical amplification feedback circuit 61, it is possible to appropriately change the optical power value of the bias light and set the optical power level of the detected light 2 to a desired value.

As described above, in the photodetector 1 according to the present invention, the optical amplification feedback circuit 6 k includes the bias light source 18 for the detected light 2.
With this configuration, in the light detection device 1 according to the present invention, the optical power of the detected light 2 and the optical power of the bias light need only reach a required level. Even if the power is small compared to the case where the optical amplification feedback circuit 6k does not have the bias light source 18 for the detected light 2, the light detection sensitivity can be improved.

(Seventh embodiment)
A block diagram showing an optical amplification feedback circuit 6m of the photodetector 1 of the seventh embodiment of the present invention is shown in FIG. Since the optical amplification feedback circuit 6m in FIG. 20 and the optical amplification feedback circuit 6 in the first embodiment in FIG. 2 are similar, changes from the optical amplification feedback circuit 6 in FIG. 2 are mainly shown.
The optical amplification feedback circuit 6m of the seventh embodiment includes an optical amplifier F for amplifying the detected light 2 before the optical amplification feedback circuit 6m. Alternatively, the optical amplification feedback circuit 6m has an optical amplifier B for amplifying the output signal light 7 at the subsequent stage of the optical amplification feedback circuit 6m. However, both the optical amplifier F and the optical amplifier B may be used.

In the optical amplification feedback circuit 6m, when the optical amplifier F is used, the optical power of the detected light 2 input to the optical amplification feedback circuit 6m can be increased without degrading the optical power detection sensitivity. The optical power of the detected light 2 can be reduced by the gain of the optical amplifier F.
In the optical amplification feedback circuit 6m, when the optical amplifier B is used, the optical power of the output signal light 7 can be increased by the gain of the optical amplifier B without degrading the optical power detection sensitivity. It has the advantage of being able to.

FIG. 21 is a block diagram showing a more specific configuration of the seventh embodiment. Since the optical amplification feedback circuit 6m in FIG. 21 is similar to the optical amplification feedback circuit 61 in the sixth embodiment in FIG. 19, the changes from the optical amplification feedback circuit 61 in FIG. 19 are mainly shown.
In the optical amplification feedback circuit 6m, the optical amplifier F is installed in the front stage of the optical amplification feedback circuit 6l in FIG. 19, and the optical amplifier B is installed in the rear stage. The optical amplifier F and the optical amplifier B are both erbium-doped fiber amplifiers EDFA, and amplify light in the C band (1530 to 1565 nm). Therefore, the optical amplifier F amplifies the 1550 nm light that is the input light to the optical amplifier F. The optical amplifier B amplifies 1558 nm light that is input light to the optical amplifier B.

As described above, the photodetector 1 according to the present invention has the optical amplifier F for amplifying the detected light 2 in the previous stage of the optical amplification feedback circuit 6m.
With this configuration, in the photodetector 1 according to the present invention, the optical power of the detected light 2 input to the optical amplification feedback circuit 6m can be increased without degrading the optical power detection sensitivity. The optical power of the light 2 can be reduced by the gain of the optical amplifier F.

Further, the photodetector 1 according to the present invention includes the optical amplifier B for amplifying the output signal light 7 at the subsequent stage of the optical amplification feedback circuit 6m.
With this configuration, in the photodetector 1 according to the present invention, the optical power of the output signal light 7 can be increased by the gain of the optical amplifier B without degrading the optical power detection sensitivity. The optical power of the light 2 can be reduced by the gain of the optical amplifier B.

  As described above, the embodiment of the present invention has been described in detail with reference to the drawings. However, the specific configuration is not limited to that described above, and various design changes and the like can be made without departing from the scope of the present invention. It is possible to

DESCRIPTION OF SYMBOLS 1 ... Photodetection device, 2 ... Light to be detected, 3 ... Photodetection element, 4 ... Electric circuit, 6, 6a, 6b, 6c, 6d, 6e, 6f, 6g, 6h, 6i, 6j, 6k, 6l, 6m DESCRIPTION OF SYMBOLS ... Optical amplification feedback circuit, 7 ... Output signal light, 11 ... Gain medium, 12 ... Optical feedback part, 13 ... Optical input part 13b, 14b ... Optical connector, 14 ... Optical output part, ORC ... Optical ring circuit, CP-P CP-A ... multiplexer, BR ... branch, OBPF ... narrow band optical transmission filter, ATT ... optical attenuator, ISO ... optical isolator, ISO-B, ISO-F ... optical isolator, 21 ... optical waveguide 21, 22, 22-1, 22-2, 22-3, 22-4 ... propagated light in circuit, OF ... optical fiber, S ... system, OS ... light source, DUT ... object to be measured, MCF ... multi-core fiber, C1, C2 ... Core, B1 ... Branch A1 ... multiplexer, 15 ... excitation source, 15d ... excitation light source, 15e ... current source, LD ... laser diode, M, M-1, M-2 ... mirror, EDF ... erbium doped fiber, 17 ... optical circulator, LS , LS1, LS2, LS3 ... lens, 18 ... bias light source, F, B ... optical amplifier, PD1, PR1 ... port

The present invention has been made to solve the above problems, and one aspect of the present invention provides a gain medium that amplifies input light to be detected, and propagation light output from the gain medium. An optical feedback device comprising: an optical feedback unit that feeds back to the optical signal; an optical amplification feedback circuit that includes the gain medium and the optical feedback unit; and a photodetector that detects incident light by converting it into a photocurrent. Then, the light generated in the optical amplification feedback circuit is output as output signal light from the optical amplification feedback circuit to be detected by the photodetecting element, and the detected light power of the amount of change in the power of the output signal light is detected. This is a light detection device in which the ratio to the amount of change is greater than 1 .
In one embodiment of the present invention, in the above-described photodetector, the power of the detected light input to the optical amplification feedback circuit is less than or equal to a laser oscillation threshold value.
In one embodiment of the present invention, in the above-described photodetector, the wavelength of the detected light input to the optical amplification feedback circuit is different from the wavelength of the output signal light.
In one embodiment of the present invention, the power of the detected light input to the optical amplification feedback circuit is greater than the power of the output signal light.

The present invention has been made to solve the above problems, and one aspect of the present invention provides a gain medium that amplifies input light to be detected, and propagation light that is output from the gain medium. An optical feedback device comprising: an optical feedback unit that feeds back to the optical signal; an optical amplification feedback circuit that includes the gain medium and the optical feedback unit; and a photodetector that detects incident light by converting it into a photocurrent. Then, the light generated in the optical amplification feedback circuit is output as output signal light from the optical amplification feedback circuit and is detected by the photodetector, and the amount of change in the value in dBm notation of the power of the output signal light is detected. In the photodetecting device, the ratio of the power of the detected light to the amount of change in the value expressed in dBm is greater than 1.
In one embodiment of the present invention, in the above-described photodetector, the power of the detected light input to the optical amplification feedback circuit is less than or equal to a laser oscillation threshold value.
In one embodiment of the present invention, in the above-described photodetector, the wavelength of the detected light input to the optical amplification feedback circuit is different from the wavelength of the output signal light.
In one embodiment of the present invention, a value in dBm notation of the power of the detected light input to the optical amplification feedback circuit is larger than a value in dBm notation of the power of the output signal light.

Claims (14)

A gain medium for amplifying the input detected light;
An optical feedback unit that feeds back propagating light output from the gain medium to the gain medium;
An optical detection device comprising: an optical amplification feedback circuit comprising the gain medium and the optical feedback unit;
An optical detection device that outputs light generated in the optical amplification feedback circuit as output signal light from the optical amplification feedback circuit. The photodetection device according to claim 1, wherein the optical feedback unit includes a narrowband light transmission filter. The photodetection device according to claim 1, wherein the light to be detected is excitation light of the gain medium. The photodetection device according to claim 1, wherein the light to be detected is signal light of the gain medium. The optical detection device according to claim 1, wherein the optical feedback unit includes an optical fiber as an optical waveguide. The photodetection device according to claim 1, wherein the optical feedback unit has a free space as an optical waveguide. The photodetection device according to claim 1, wherein the optical feedback unit has a ring shape. The photodetection device according to claim 1, wherein the optical feedback unit has a linear shape. The optical feedback unit has an optical circulator,
The light detection apparatus according to claim 1, wherein the light to be detected and the output signal light are output through the optical circulator. The photodetection device according to claim 1, wherein the optical amplification feedback circuit includes a bias light source for the detected light. The photodetector according to claim 1, wherein the gain medium is a rare earth-doped fiber. The photodetection device according to claim 1, wherein the gain medium is a laser diode. The photodetection device according to claim 1, further comprising an optical amplifier for amplifying the detected light before the optical amplification feedback circuit. The photodetection device according to claim 1, further comprising an optical amplifier for amplifying the output signal light at a stage subsequent to the optical amplification feedback circuit.

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