Photomixer

Abstract

Provided is a photomixer capable of achieving at least one of reduced size and increases output of an electric signal. A photomixer 10 according to the present disclosure comprises a light receiving unit 11 that receives incident light and converts the light into an electric signal having a predetermined frequency, an amplification unit 12 that amplifies the electric signal output from the light receiving unit 11, and a coaxial connector 13 that receives and outputs the electric signal amplified by the amplification unit 12.

Description

Photomixer

[0001] The present disclosure relates to a photomixer.

[0002] In recent years, technologies related to Beyond 5G have been attracting attention as technologies to be used in next-generation high-speed, large-capacity networks. Beyond 5G requires further improvements and advancements in key performance of 5G wireless, for example, by 10 times or more. Key performance includes, for example, communication speed, latency, and simultaneous connections. For example, Beyond 5G requires ultra-high speeds exceeding 10 Gbps / ch, ultra-low latency of about 100 μs, and 100 ch / m. ２ There is a demand for a massive number of simultaneous connections.

[0003] To achieve ultra-high speeds in Beyond 5G, wireless carrier frequencies can be shifted to the terahertz band, which includes frequencies from 100 GHz to 10 THz, making it easier to secure communication bandwidth. This makes it possible to achieve ultra-high speeds of 50 Gbps / ch. In recent years, proposals for systems that combine optical fiber and wireless communication have been reported, and conventional technology is also known that generates THz waves from two-tone signals in optical fiber using the principle of a photomixer.

[0004] Conventionally, electromagnetic waves in the 0.1 to 10 THz spectral band are defined as THz waves, and photomixers using photodiodes have been known as devices for generating THz waves. For example, Patent Document 1 discloses a photomixer that can efficiently utilize the optical input power of an input optical signal to generate millimeter-wave and terahertz-wave electromagnetic waves.

[0005] JP 2013-070210 A

[0006] However, in conventional technology, a waveguide connector is used to output terahertz waves from the photomixer, resulting in a large package size. In addition, an amplifier circuit for amplifying the electrical signal output from the photodiode is not installed, resulting in a small electrical signal output. Conventional photomixers have room for improvement in terms of miniaturization and increased electrical signal output.

[0007] An object of the present disclosure is to provide a photomixer that can achieve at least one of miniaturization and increased electrical signal output.

[0008] The means for achieving the above objectives are as follows:

[0009] (1) A photomixer comprising: a light receiving unit that receives incident light and converts it into an electrical signal having a predetermined frequency; an amplifier that amplifies the electrical signal output from the light receiving unit; and a coaxial connector that receives and outputs the electrical signal amplified by the amplifier.

[0010] (2) The photomixer according to (1) above, further comprising: a pair of ground lines continuously connected from the light receiving unit to the coaxial connector; and a signal line continuously connected between the pair of ground lines from the light receiving unit to the coaxial connector.

[0011] (3) The photomixer according to (1) or (2) above, wherein the light receiving unit includes a photodiode that receives the light and outputs the electrical signal, and a termination resistor that connects the photodiode to ground.

[0012] (4) The photomixer according to any one of (1) to (3) above, further comprising a waveguide section disposed between the amplifier section and the coaxial connector, for transmitting the electrical signal output from the amplifier section to the coaxial connector.

[0013] (5) The photomixer according to (4) above, further comprising a metal package in which the light receiving section, the amplifier section, and the waveguide section are built in by hybrid mounting.

[0014] (6) The photomixer according to any one of (1) to (5) above, wherein the light includes two frequency components spaced apart from each other at the predetermined frequency, and the light receiving unit outputs the electrical signal as a beat signal of the predetermined frequency.

[0015] (7) The photomixer according to any one of (1) to (6) above, wherein the predetermined frequency is included in a frequency band of 100 GHz or higher.

[0016] According to the present disclosure, it is possible to provide a photomixer that can achieve at least one of miniaturization and increased output of an electrical signal.

[0017] 1 is a schematic diagram showing an example of the configuration of an optical system including a photomixer according to an embodiment of the present disclosure; FIG. 2 is a circuit diagram showing a first example of the outline of the configuration of the photomixer of FIG. 1; FIG. 3 is an implementation diagram showing a second example of the outline of the configuration of the photomixer of FIG. 1; FIG. 4 is an enlarged view of an area IV enclosed by a dashed dotted line in FIG. 3; FIG. 5 is a graph showing an example of the spectrum of light incident on the photomixer of FIG. 1; and FIG. 6 is a graph showing an example of the spectrum of an electrical signal output from the photomixer of FIG. 1.

[0018] Hereinafter, one embodiment of the present disclosure will be mainly described with reference to the accompanying drawings.

[0019] 1 is a schematic diagram showing an example of the configuration of an optical system 1 including a photomixer 10 according to an embodiment of the present disclosure. With reference to FIG. 1, an example of the configuration and function of the optical system 1 that generates an optical two-tone signal and inputs it to the photomixer 10 will be mainly described. In addition to the photomixer 10 arranged on the light receiving side, the optical system 1 includes a light source unit 20, a modulation unit 30, a signal generator 40, a filter unit 50, a branching unit 60, and a monitor unit 70.

[0020] The optical system 1 generates an optical two-tone signal including two frequency components spaced apart at a predetermined frequency, and inputs the signal to the photomixer 10. The optical system 1 receives the generated optical two-tone signal using the photomixer 10, and outputs an RF (Radio Frequency) signal from the photomixer 10 as a beat signal of the predetermined frequency.

[0021] In the present disclosure, the "predetermined frequency" may be included in a frequency band of, for example, 100 GHz or higher. Although 100 GHz is given as an example of the lower limit of the frequency band, the lower limit is not limited to the exact value of "100 GHz" with a 0% error. The lower limit may include a predetermined error. The "predetermined error" may preferably include an error of 1% or less, more preferably 0.1% or less, more preferably 0.01% or less, and even more preferably 0.001% or less.

[0022] In addition, the "predetermined frequency" may be included in a frequency band of, for example, 10 THz or less. Although 10 THz is given as an example of the upper limit of the frequency band, the upper limit is not limited to the exact value of "10 THz" with a 0% error. The upper limit may include a predetermined error. The "predetermined error" may preferably include an error of 1% or less, more preferably 0.1% or less, more preferably 0.01% or less, and even more preferably 0.001% or less.

[0023] The light source unit 20 irradiates light, which is the source of the optical two-tone signal in the optical system 1, toward the modulation unit 30. The light source unit 20 has a laser light source including, for example, a semiconductor laser (laser diode: LD). The light irradiated from the light source unit 20 has any wavelength that can be used for optical communication in the optical system 1. The wavelength of the light irradiated from the light source unit 20 is, for example, included in the 1.5 μm optical communication wavelength band. However, the wavelength of the light irradiated from the light source unit 20 may be, for example, included in another near-infrared region different from the 1.5 μm optical communication wavelength band, another infrared region, a visible region, an ultraviolet region, or the like.

[0024] The modulation unit 30 receives the light emitted from the light source unit 20 and performs intensity modulation or phase modulation on the light. ３The modulator 30 includes a Mach-Zehnder type optical modulator using a crystal. The modulator 30 receives an RF signal output from a signal generator 40 and generates a plurality of sidebands centered on a carrier wave having a frequency fc from the light emitted from the light source 20. For example, the modulator 30 generates a first sideband having a frequency f1 greater than the frequency fc and a second sideband having a frequency f2 less than the frequency fc. The frequency difference f1-f2 between the first sideband and the second sideband corresponds to a predetermined frequency.

[0025] The signal generator 40 is electrically connected to the optical modulator of the modulation unit 30 and outputs an RF signal to the optical modulator. The signal generator 40 includes, for example, a function generator. The signal generator 40 outputs an RF signal having a frequency of f=(f1-f2) / 2 to the optical modulator of the modulation unit 30 in order to generate a first sideband and a second sideband with a frequency difference f1-f2 corresponding to a predetermined frequency in the light irradiated from the light source unit 20.

[0026] In this case, the frequency f1 of the first sideband is fc + (f1 - f2) / 2. The frequency f2 of the second sideband is fc - (f1 - f2) / 2. Since fc = (f1 + f2) / 2, the frequency fc of the carrier wave is located at the center between the frequency f1 of the first sideband and the frequency f2 of the second sideband.

[0027] The filter unit 50 extracts only the first and second sideband components from the light generated by the modulator 30. The filter unit 50 attenuates the carrier wave of frequency fc to a very low level and passes the first and second sidebands having frequencies f1 and f2, respectively, almost unchanged.

[0028] The filter unit 50 includes, for example, a bandstop filter and a bandpass filter. The bandstop filter attenuates light to a very low level only in a predetermined frequency band centered around frequency fc, while passing light almost unchanged in other frequency bands. The bandpass filter passes light almost unchanged only in predetermined frequency bands centered around frequencies f1 and f2, while attenuating light to a very low level in other frequency bands.

[0029] The branching unit 60 guides a portion of the power of the optical two-tone signal having the first sideband and the second sideband extracted by the filter unit 50 to the photomixer 10, and guides the other portion to the monitor unit 70. The branching unit 60 includes, for example, a 10 dB optical coupler and a 3 dB optical coupler. The branching unit 60 guides most of the power of the optical two-tone signal to the photomixer 10 by using, for example, the 10 dB optical coupler, and guides the other portion to the monitor unit 70 for power monitoring.

[0030] The monitor unit 70 monitors the power of a portion of the optical two-tone signal branched by the branch unit 60. The monitor unit 70 includes, for example, an optical power meter.

[0031] Fig. 2 is a circuit diagram showing a first example of the schematic configuration of the photomixer 10 of Fig. 1. In Fig. 2, a waveguide section 14, a ground line 15, a signal line 16, a metal package 17, and the like of the photomixer 10, which will be described later, are omitted for the purpose of simplifying the illustration. The first example of the schematic configuration of the photomixer 10 according to an embodiment of the present disclosure will be mainly described with reference to Fig. 2.

[0032] A photomixer 10 according to one embodiment receives an optical two-tone signal generated in the optical system 1 and outputs an RF signal as a beat signal of a predetermined frequency. The photomixer 10 includes a light receiving unit 11, an amplifier unit 12, and a coaxial connector 13.

[0033] The light receiving unit 11 receives incident light and converts it into an electrical signal having a predetermined frequency. The light includes two frequency components spaced apart by the predetermined frequency. The light receiving unit 11 receives a portion of the optical two-tone signal branched by the branching unit 60 of the optical system 1 shown in FIG. 1. The light receiving unit 11 outputs an electrical signal as a beat signal of the predetermined frequency. The light receiving unit 11 includes a photodiode 111 that receives light and outputs an electrical signal, and a termination resistor 112 that connects the photodiode 111 to ground.

[0034] The photodiode 111 includes an ultra-wideband photoelectric conversion element such as a UTC-PD (Uni-Traveling Carrier Photodiode). A DC (Direct Current) bias voltage Vpd is applied to one end of the photodiode 111. A termination resistor 112 and an amplifier unit 12 are connected in parallel to the other end of the photodiode 111. The termination resistor 112 connected to the other end of the photodiode 111 includes, for example, a 50 Ω resistor. One end of the termination resistor 112 is connected to the photodiode 111 and the other end is connected to ground. The light receiving unit 11 is configured, for example, by a UTC-PD with an output 50 Ω termination resistor capable of achieving a frequency band of 100 GHz or more.

[0035] The amplifier 12 amplifies the electrical signal output from the light receiving unit 11. The amplifier 12 receives the electrical signal output by photoelectric conversion of the light received by the photodiode 111 of the light receiving unit 11 and amplifies the signal so as to increase the signal-to-noise (S / N) ratio. The amplifier 12 has an amplifier circuit 121 that amplifies the electrical signal, a first capacitor 122 arranged on the input side of the amplifier circuit 121, and a second capacitor 123 arranged on the output side of the amplifier circuit 121.

[0036] The amplifier circuit 121 includes, for example, an integrated circuit of a low-noise element such as a high electron mobility transistor (HEMT) or a heterojunction bipolar transistor (HBT) using an InP (indium phosphate) or GaAs (gallium arsenide) compound semiconductor process. The amplifier circuit 121 is connected to the other end of the photodiode 111 of the light receiving unit 11 via a first capacitor 122 for alternating current (AC) coupling. The amplifier circuit 121 receives an input of an electrical signal as an RF signal from the photodiode 111 via the first capacitor 122. The amplifier circuit 121 is similarly connected to the coaxial connector 13 via a second capacitor 123. The amplifier circuit 121 amplifies the electrical signal as an RF signal and outputs it to the coaxial connector 13 via the second capacitor 123.

[0037] The amplifier circuit 121 is configured such that a first terminal of a transistor included in the amplifier circuit 121 is connected to ground. The amplifier circuit 121 is configured such that a gate voltage Vg is applied to a second terminal of the transistor included in the amplifier circuit 121. The amplifier circuit 121 is configured such that a DC bias voltage Vd is applied to a third terminal of the transistor included in the amplifier circuit 121.

[0038] The coaxial connector 13 receives and outputs the electrical signal amplified by the amplifier unit 12. The coaxial connector 13 is a connector for reducing the package size of the photomixer 10, and unlike conventional waveguide connectors, is a connector with a coaxial structure having a band characteristic of DC-100 GHz or more. The coaxial connector 13 receives the electrical signal as an RF signal amplified by the amplifier circuit 121 of the amplifier unit 12 and output via the second capacitor 123, and outputs it to the outside of the photomixer 10. The electrical signal output from the coaxial connector 13 to the outside of the photomixer 10 is guided through a flexible coaxial cable to another electro-optical component different from the photomixer 10.

[0039] Fig. 3 is a mounting diagram showing a second example of the schematic configuration of the photomixer 10 of Fig. 1. Fig. 4 is an enlarged view of the area IV enclosed by the dashed dotted line in Fig. 3. In Figs. 3 and 4, the circuit elements of the components specifically shown in Fig. 2, the coaxial connector 13, and the like are omitted for the purpose of simplifying the illustration. The second example of the schematic configuration of the photomixer 10 according to an embodiment of the present disclosure will be mainly described with reference to Figs. 3 and 4.

[0040] The photomixer 10 according to the embodiment further includes a waveguide section 14 , a ground line 15 , a signal line 16 , and a metal package 17 in addition to the light receiving section 11 , the amplifier section 12 , and the coaxial connector 13 .

[0041] 4, the waveguide section 14 is disposed between the amplifier section 12 and the coaxial connector 13, and transmits the electrical signal output from the amplifier section 12 to the coaxial connector 13. The waveguide section 14 is disposed on the output side of the photomixer 10, opposite the light-receiving section 11, which is located on the input side of the amplifier section 12.

[0042] The waveguide unit 14 includes, for example, a grounded coplanar waveguide (G-CPWG) formed on a ceramic 143. The grounded coplanar waveguide of the waveguide unit 14 includes, for example, a signal line 142 formed on the ceramic 143, and ground lines 141 formed on the ceramic 143 so as to sandwich the signal line 142 from both sides.

[0043] The pair of ground lines 15 are connected continuously from the light receiving unit 11 to the coaxial connector 13. For example, each of the pair of ground lines 15 is positioned outside the signal line 16 and connected continuously to a plurality of different components arranged up to the coaxial connector 13, including the light receiving unit 11, the amplifier unit 12, and the waveguide unit 14.

[0044] Each of the pair of ground lines 15 is configured by electrically connecting the ground terminal pad 113 of the light receiving unit 11 to the ground terminal pad 124 of the amplifier unit 12 by wire bonding or soldering. Each of the pair of ground lines 15 is configured by electrically connecting the ground terminal pad 124 of the amplifier unit 12 to the ground line 141 of the waveguide unit 14 by wire bonding or soldering.

[0045] The signal line 16 is continuously connected between the pair of ground lines 15 from the light receiving unit 11 to the coaxial connector 13. For example, the signal line 16 is sandwiched between the pair of ground lines 15 and continuously connected to a plurality of different components arranged up to the coaxial connector 13, including the light receiving unit 11, the amplifier unit 12, and the waveguide unit 14.

[0046] The signal line 16 is configured by electrically connecting the signal terminal pad 114 of the light receiving unit 11 and the signal terminal pad 125 of the amplifier unit 12 by wire bonding or soldering. The signal line 16 is configured by electrically connecting the signal terminal pad 125 of the amplifier unit 12 and the signal line 142 of the waveguide unit 14 by wire bonding or soldering.

[0047] 3, the metal package 17 incorporates the light receiving unit 11, the amplifier unit 12, and the waveguide unit 14 through hybrid mounting. Although only one part of the metal package 17 is shown in FIG. 3, in reality, the metal package 17 is configured as a housing that combines at least two parts and houses each component of the photomixer 10 inside.

[0048] The coaxial connector 13 is attached to the waveguide portion 14 by electrically connecting the signal line of the coaxial connector 13 to the signal line 142 of the waveguide portion 14 and electrically connecting the ground line of the coaxial connector 13 to the ground line 141 of the waveguide portion 14. When attached to the waveguide portion 14, the coaxial connector 13 is fixed to the metal package 17 as a housing so as to protrude outward from the side surface of the metal package 17.

[0049] As described above, the photomixer 10 is miniaturized by incorporating, for example, a transmission line with electrode wiring design into the metal package 17 using hybrid mounting technology. The photomixer 10 uses wire bonding or soldering for signal electrical connection, and outputs an electrical signal from the coaxial connector 13.

[0050] Fig. 5 is a graph showing an example of the spectrum of light incident on the photomixer 10 of Fig. 1. Fig. 5 shows an example of the spectrum of an optical two-tone signal generated by the optical system 1 of Fig. 1. In Fig. 5, the horizontal axis represents wavelength, and the vertical axis represents spectral intensity. As an example, the optical two-tone signal may have a spectral intensity as shown in Fig. 5 in the optical communication wavelength band of 1.5 μm.

[0051] The optical two-tone signal has a first sideband and a second sideband with a frequency difference Δf=f1-f2 corresponding to a predetermined frequency of 100 GHz. When the predetermined frequency of 100 GHz is converted into a wavelength, the first sideband and the second sideband have a wavelength difference of 0.8 nm, which corresponds to the interval indicated by the arrow in FIG. 5.

[0052] Fig. 6 is a graph showing an example of the spectrum of the electrical signal output from the photomixer 10 of Fig. 1. Fig. 6 shows an example of the spectrum of the electrical signal as an RF signal amplified by the amplifier 12 of Fig. 2 and output from the coaxial connector 13. In Fig. 6, the horizontal axis represents frequency, and the vertical axis represents spectral intensity.

[0053] As an example, the electrical signal was obtained as an RF signal with a center frequency of 99.9999717 GHz and a spectrum with a very high signal-to-noise ratio. The noise of the electrical signal was -63.74 dBm / Hz. In this way, the photomixer 10 was able to output an electrical signal having a predetermined frequency of approximately 100 GHz from the optical two-tone signal.

[0054] The photomixer 10 according to the embodiment described above can achieve at least one of miniaturization and increased electrical signal output. For example, the photomixer 10 can achieve both miniaturization and increased electrical signal output.

[0055] For example, the photomixer 10 has a coaxial connector 13 that receives and outputs the electrical signal amplified by the amplifier unit 12. This allows the photomixer 10 to have a smaller connector size and a smaller package size compared to conventional photomixers that use waveguide connectors. Therefore, the photomixer 10 can be miniaturized. In addition, the coaxial connector 13 allows the photomixer 10 to be wired using a flexible coaxial cable, thereby improving the flexibility of placement when incorporated into devices, systems, etc. The mounting efficiency of the photomixer 10 is improved. As a result, the convenience of the photomixer 10 is improved.

[0056] For example, the photomixer 10 has an amplifier 12 that amplifies the electrical signal output from the light-receiving unit 11. As a result, the photomixer 10 incorporates an amplifier circuit 121 that amplifies the electrical signal output from the light-receiving unit 11, thereby improving the output of the electrical signal. Compared to conventional photomixers that do not include an amplifier circuit, the photomixer 10 can improve the S / N ratio of the electrical signal. Therefore, the photomixer 10 can increase the output of the electrical signal.

[0057] The photomixer 10 has a pair of ground lines 15 that are continuously connected from the light receiving unit 11 to the coaxial connector 13, and a signal line 16 that is continuously connected between the pair of ground lines 15 from the light receiving unit 11 to the coaxial connector 13.

[0058] As a result, the photomixer 10 can obtain stable output characteristics for electrical signals simply by connecting the light receiving unit 11 and the amplifier unit 12, and between the amplifier unit 12 and the coaxial connector 13, in the order of ground-signal-ground. The photomixer 10 facilitates the implementation of each component, thereby stabilizing the output characteristics of electrical signals. For example, in conventional photomixers that use waveguide connectors, the output characteristics tend to be sensitive to changes depending on the implementation state of the wire bonding between the photodiode and the waveguide connector. The photomixer 10 according to one embodiment can obtain stable output characteristics compared to such conventional technology, thereby improving convenience.

[0059] The light-receiving unit 11 includes a photodiode 111 that receives light and outputs an electrical signal, and a termination resistor 112 that connects the photodiode 111 to ground. This allows the photomixer 10 to arrange a DC current path and pass a DC current through the photodiode 111 of the light-receiving unit 11, even if the frequency band of the amplifier circuit 121 of the amplifier unit 12 does not include DC. Therefore, the photomixer 10 can operate the photodiode 111 of the light-receiving unit 11 normally and easily obtain an electrical signal from the optical two-tone signal incident on the photodiode 111.

[0060] The photomixer 10 is disposed between the amplifier 12 and the coaxial connector 13 and has a waveguide 14 that transmits the electrical signal output from the amplifier 12 to the coaxial connector 13. This allows the photomixer 10 to stably guide the electrical signal amplified by the amplifier 12 to the coaxial connector 13. Even if it is difficult to directly connect the amplifier circuit 121 of the amplifier 12 to the coaxial connector 13, the photomixer 10 can easily achieve electrical connection between the amplifier 12 and the coaxial connector 13 via the waveguide 14.

[0061] The photomixer 10 has a metal package 17 in which the light receiving section 11, the amplifier section 12, and the waveguide section 14 are built in by hybrid mounting. As a result, the photomixer 10 can be miniaturized by accommodating each component inside the metal package 17 through hybrid mounting. The photomixer 10 can be easily miniaturized by continuously mounting each component along three lines: ground, signal, and ground. By miniaturizing the photomixer 10, the degree of freedom in placement when it is incorporated into devices, systems, etc. is improved. The mounting efficiency of the photomixer 10 is improved. As a result, the convenience of the photomixer 10 is improved.

[0062] The light includes two frequency components spaced apart by a predetermined frequency. The light-receiving unit 11 outputs an electrical signal as a beat signal of the predetermined frequency. This allows the photomixer 10 to easily obtain an electrical signal of the predetermined frequency from the optical two-tone signal.

[0063] The predetermined frequency is included in the frequency band of 100 GHz or higher. This enables the photomixer 10 to operate in the terahertz band, which includes frequencies from 100 GHz to 10 THz. The photomixer 10 can also contribute to the generation of terahertz electromagnetic waves by efficiently utilizing the optical input power of the input optical signal.

[0064] It will be apparent to those skilled in the art that the present disclosure may be embodied in other specific forms other than the above-described embodiments without departing from the spirit or essential characteristics thereof. Therefore, the foregoing description is illustrative and not limiting. The scope of the disclosure is defined not by the foregoing description but by the appended claims. All modifications within the range of equivalents of any modifications are intended to be embraced therein.

[0065] For example, the shape, pattern, size, arrangement, orientation, type, and number of each of the above-described components are not limited to those shown in the above description and drawings. The shape, pattern, size, arrangement, orientation, type, and number of each component may be configured arbitrarily as long as the function can be realized. Each component of the illustrated optical system 1 and photomixer 10 is functionally conceptual. The specific form of each component is not limited to that shown in the drawings.

[0066] In the above embodiment, the photomixer 10 has been described as having the amplifier 12 that amplifies the electrical signal output from the light receiving unit 11, but this is not limiting. The photomixer 10 does not have to have the amplifier 12. Even in such a case, the photomixer 10 can be made smaller as long as it has the coaxial connector 13. Therefore, the photomixer 10 can achieve both miniaturization and increased electrical signal output, with the latter being more compact.

[0067] In the above embodiment, the photomixer 10 has been described as having a coaxial connector 13 that receives and outputs the electrical signal amplified by the amplifier 12, but this is not limiting. The photomixer 10 may have a waveguide connector instead of or in addition to the coaxial connector 13. Even in such a case, the photomixer 10 can improve the output of the electrical signal as long as it has the amplifier 12. Therefore, the photomixer 10 can achieve increased output of the electrical signal, out of the two possible combinations of miniaturization and increased output of the electrical signal.

[0068] In the above embodiment, the photomixer 10 has been described as having a pair of ground lines 15 that are continuously connected from the light-receiving unit 11 to the coaxial connector 13, but this is not limited to this. The ground lines 15 of the photomixer 10 do not have to be continuously connected from the light-receiving unit 11 to the coaxial connector 13. The photomixer 10 may have a number of ground lines 15 other than a pair. Similarly, the photomixer 10 has been described as having a signal line 16 that is continuously connected between the pair of ground lines 15 from the light-receiving unit 11 to the coaxial connector 13, but this is not limited to this. The signal line 16 may be arranged in any manner relative to any number of ground lines 15.

[0069] In the above embodiment, the light receiving unit 11 has been described as including the photodiode 111 that receives light and outputs an electrical signal, and the termination resistor 112 that connects the photodiode 111 to ground, but is not limited to this. For example, if the frequency band of the amplifier circuit 121 of the amplifier 12 includes DC and a DC current can be passed through the photodiode 111 of the light receiving unit 11, the light receiving unit 11 does not need to include the termination resistor 112.

[0070] In the above embodiment, the photodiode 111 is described as including a UTC-PD, but is not limited to this. The photodiode 111 may also include any other photoelectric conversion element. The photodiode 111 may also include any other photoelectric conversion element that uses waveguide-type InGaAs (indium gallium arsenide) as an absorption layer. The termination resistor 112 connected to the other end of the photodiode 111 is described as including a 50 Ω resistor, but is not limited to this. The termination resistor 112 may also include a resistor having any other resistance value as long as it can achieve its function.

[0071] In the above embodiment, the photomixer 10 has been described as being disposed between the amplifier 12 and the coaxial connector 13 and having the waveguide 14 that transmits the electrical signal output from the amplifier 12 to the coaxial connector 13, but this is not limiting. The photomixer 10 does not have to have the waveguide 14. In the photomixer 10, the amplifier 12 and the coaxial connector 13 may be directly connected to each other without the waveguide 14.

[0072] In the above embodiment, the photomixer 10 has been described as having a metal package 17 in which the light receiving unit 11, the amplifier unit 12, and the waveguide unit 14 are built in by hybrid mounting, but this is not limiting. The photomixer 10 may have a housing made of a material other than the metal package 17.

[0073] In the above embodiment, the light includes two frequency components spaced apart by a predetermined frequency, and the light receiving unit 11 outputs an electrical signal as a beat signal of the predetermined frequency. However, the present invention is not limited to this. The photomixer 10 may generate an electrical signal of the predetermined frequency from an optical signal by any other method.

[0074] In the above embodiment, the predetermined frequency is described as being included in the frequency band of 100 GHz or higher, but is not limited thereto, and may be included in a frequency band of less than 100 GHz.

[0075] REFERENCE SIGNS LIST 1 Optical system 10 Photomixer 11 Light receiving section 111 Photodiode 112 Termination resistor 113 Ground terminal pad 114 Signal terminal pad 12 Amplifying section 121 Amplifying circuit 122 First capacitor 123 Second capacitor 124 Ground terminal pad 125 Signal terminal pad 13 Coaxial connector 14 Waveguide section 141 Ground line 142 Signal line 143 Ceramics 15 Ground line 16 Signal line 17 Metal package 20 Light source section 30 Modulation section 40 Signal generator 50 Filter section 60 Branching section 70 Monitor section

Claims

1. A photomixer comprising: a light receiving unit that receives incident light and converts it into an electrical signal having a predetermined frequency; an amplifier that amplifies the electrical signal output from the light receiving unit; and a coaxial connector that receives and outputs the electrical signal amplified by the amplifier.

2. A photomixer according to claim 1, further comprising: a pair of ground lines continuously connected from the light receiving unit to the coaxial connector; and a signal line continuously connected between the pair of ground lines from the light receiving unit to the coaxial connector.

3. A photomixer according to claim 1 or 2, wherein the light receiving section has a photodiode that receives the light and outputs the electrical signal, and a termination resistor that connects the photodiode to ground.

4. A photomixer according to claim 1 or 2, further comprising a waveguide section disposed between said amplifier section and said coaxial connector, for transmitting said electrical signal output from said amplifier section to said coaxial connector.

5. A photomixer according to claim 4, further comprising a metal package in which the light receiving section, the amplifier section, and the waveguide section are built in by hybrid mounting.

6. A photomixer according to claim 1 or 2, wherein the light includes two frequency components spaced apart from each other at the predetermined frequency, and the light receiving section outputs the electrical signal as a beat signal of the predetermined frequency.

7. A photomixer according to claim 1 or 2, wherein the predetermined frequency is included in a frequency band of 100 GHz or higher.

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