Multiplication and amplification of an optical signal

Abstract

A signal multiplication device includes a signal source that generates a bias voltage and a signal source that generates an optical signal. The device also includes an electron injection photodetector that receives the bias voltage and the optical signal. The electron injection photodetector multiplies the bias voltage and the optical signal to generate an output photocurrent that is dependent on the bias voltage.

Description

MULTIPLICATION AND AMPLIFICATION OF AN OPTICAL SIGNALCROSS-REFERENCE TO RELATED APPLICATION

[0001] The present application claims the priority benefit of U.S. Provisional Patent App. No. 63 / 531,115 filed on August 7, 2023, the entire disclosure of which is incorporated by reference herein.BACKGROUND

[0002] Systems such as optical coherence tomography (OCT) utilize optical signals to generate images of a desired sample, such as a retina. Similarly, light detection and ranging (LIDAR) systems utilize optical signals to determine distance / range of an object from a given location. The depth at which systems are able to obtain measurements is based on a frequency of the optical signal being used to perform the measurement. Specifically, higher frequency optical signals result in an abi li ty to measure greater depths. However, the high frequency components required to generate high frequency signals are expensive and bulky, as compared to lower frequency components. As a result, it can be difficult to obtain high depth measurements with a system that is cost effective and small in size.SUMMARY

[0003] An illustrative signal multiplication device includes a signal source that generates a bias voltage and a signal source that generates an optical signal. The device also includes an electron injection photodetector that receives the bias voltage and the optical signal. The electron injection photodetector multiplies the bias voltage and the optical signal to generate an output photocurrent that is dependent on the bias voltage.

[0004] In one embodiment, the signal source that generates the optical signal comprises a laser of a light detection and ranging system. In another embodiment, the system includes a local oscillator, where a frequency of the local oscillator is set to downshift optical signal frequency. In another embodiment, the electron injection photodetector performs frequency shifting based on the multiplication of the bias voltage and the optical signal. In another embodiment, the output photocurrent corresponds to a signal having a first frequency that is able to take a first depth measurement of a sample, where the optical signal has a second frequency that is less than the first frequency.

[0005] In one embodiment, the bias voltage is set to take the electron injection photodetector out of a saturation region, and into a region where responsivity of the electron injection photodetector is a linear function of bias voltage. In another embodiment, the output photocurrent comprises a product of absorbed power from the optical signal and the responsivity of the electron injection photodetector. In an illustrative embodiment, the electron injection photodetector includes a type II heterojunction. In another embodiment, the signal source that generates the optical signal comprises a swept-source laser of an optical coherence tomography system. In another embodiment, the device includes a bias tee that is configured to receive and distribute the bias voltage. In another embodiment, the device includes a diplexer that is electrically connected to both the bias tee and the electron injection photodetector.

[0006] An illustrative method for performing signal multiplication includes receiving, by an electron injection photodetector, a bias voltage signal. The method also includes receiving, by the electron injection photodetector, an optical signal. The method further includes multiplying, by the electron injection photodetector, the bias voltage signal and the optical signal to generate an output photocurrent that is dependent on the bias voltage signal.

[0007] In one embodiment, the method includes generating the optical signal by a swept- source laser of an optical coherence tomography system. In another embodiment, the method includes generating, by a light detection and ranging system, the optical signal. In one embodiment, the method also includes setting a frequency of a local oscillator, wherein the frequency of the local oscillator is set to downshift the optical signal. In another embodiment, the method includes performing, by the electron injection photodetector, frequency shifting based on the multiplication.

[0008] In one embodiment, the output photocurrent corresponds to a signal having a first frequency that is able to take a first depth measurement of a sample, and the optical signal has a second frequency that is less than the first frequency. In another embodiment, the method includes receiving and distributing the bias voltage signal by a bias tee. In another embodiment, the device includes a diplexer that is electrically connected to both the bias tee and the photodetector.

[0009] Other principal features and advantages of the invention w ill become apparent to those skilled in the art upon review of the following drawings, the detailed description, and the appended claims.BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Illustrative embodiments of the invention will hereafter be described with reference to the accompanying drawings, wherein like numerals denote like elements.

[0011] Fig. 1 is a block diagram depicting components of the test system to demonstrate signal multiplication in accordance with an illustrative embodiment.

[0012] Fig. 2 depicts the convolution of two signal spikes in the form of a broader spike in accordance with an illustrative embodiment.

[0013] Fig. 3 depicts measured power of the peak in accordance with an illustrative embodiment.

[0014] Fig. 4 is a graph depicting optical fluctuation versus optical power in accordance with an illustrative embodiment.

[0015] Fig. 5A displays the working principle behind the multiplication capability of electron injection photodetector / phototransistor devices in accordance with an illustrative embodiment.

[0016] Fig. 5B is a graph depicting responsivity (y-axis) versus the bias voltage (x-axis) applied to the electron injection photodetector device in accordance with an illustrative embodiment.

[0017] Fig. 5C depicts a layer structure of an electron injection photodetector including the layer material, the layer thickness, and the dopant type and concentration for each layer in accordance with an illustrative embodiment.

[0018] Fig. 6 depicts an OCT system that incorporates an electron injection phototransistor to facilitate signal multiplication in accordance with an illustrative embodiment.

[0019] Fig. 7 depicts a computing device for performing signal multiplication in accordance with an illustrative embodiment.DETAILED DESCRIPTION

[0020] Described herein are methods and systems for multiplication of an optical signal with another electrical and / or optical signal using an electron injection photodetector or a type-II heterojunction photodetector. Multiplication of signals is an important operation,since it benefits a myriad of applications in which operations such as frequency mixing, signal processing, neuromorphic computing, heterodyning, homodyning, and frequency shifting are desired. An electron injection photodetector can achieve such multiplication and the inventors have experimentally demonstrated this ability.

[0021] Two important application examples of this ability’ are in the field of in light detection and ranging (LiDAR) systems and optical coherence tomography (OCT), where this method could be used to significantly reduce the downstream bandwidth of the electronics and hence the total cost, weight, power, and complexity (a.k.a. "SWaP") of the system. In the OCT field, for example, signal multiplication can be used to achieve high-resolution images at vastly different depths (say the front of the eye and the retina) almost simultaneously. In the field of LiDAR, for example, this ability could be used to code and demodulate the signal to increase the signal immunity to noise and interference. It could also be used for increased range of detection.

[0022] The inventors have run some tests in the lab to verify the frequency mixing capabilities of electron injection phototransistors with good results. In terms of overall equipment, the test system makes use of the following components: a 50 micrometer (urn) diameter heterojunction phototransistor, a 50 ohm shunt resistor, a 6 gigaHertz (GHZ) bandwidth bias tee, a 7GHz bandwidth spectrum analyzer, a 2-channel direct current (DC) power supply, a 6.5-digit digital multimeter, a diplexer with a LP of DC-10 megaHertz (MHz) and a HP of 40MHz-2. 15 GHz, a signal generator with 6GHz bandwidth, to act as a local oscillator (LO), a distributed feedback (DFB) (single-mode) laser operating at 1550 nanometer (nm) wavelength, with a maximum output of 12 milliWatts (mW), a Mach Zehnder Modulator (MZM) with 40GHz bandwidth, a signal generator with 2GHz bandwidth, to act as an input for the MZM, a fiber polarizer, a fiber attenuator, a fiber 2x2 coupler, a fiber-coupled infrared (IR)-sensitive PIN detector, a handheld multimeter, a collimator / beam expander on a kinematic mount, a fiber-coupled broadband illuminator, two beam splitters, an infrared (IR) objective lens on a kinematic mount, and a 1550nm w avelength sensitive camera with a 4f lens system mounted in front of it. Fig. 1 is a block diagram depicting components of the test system to demonstrate signal multiplication in accordance w ith an illustrative embodiment.

[0023] The experimental procedure for a primary proof-of-concept of signal multiplication was as follows. The illuminator and camera were turned on in order to imagethe phototransistor chip and find the candidate phototransistor device. The laser was turned on, and the location of the back-reflected laser light on the camera image plane was noted. The laser beam was roughly aligned by using the IR objective’s kinematic mount to make the approximate location of the beam on the camera image plane collinear with the optically active area of the phototransistor.

[0024] Final alignment of the phototransistor was conducted by DC biasing the phototransistor to 2. IV and using the current readout of the 6.5 digit DMM to optimize the photoresponse. The IR objective kinematics were adjusted, the collimator tip / tilt was adjusted, fiber attenuation was adjusted, MZM DC bias was adjusted, and polarization was adjusted. A DC photocurrent of 10 mA was attained in this case. The DC photocurrent of the fiber-coupled PIN was measured with the handheld multimeter and monitored continuously throughout the experiment to track any optical power drift. Based on prior calibrations, the fiber-coupled PIN always receives an approximately 3.5 decibel (dB) stronger signal than the device being coupled to from the free-space objective. Additionally, ~130uW of optical pow er was detected, meaning that ~60uW of optical power was incident on the phototransistor. This means the phototransistor responsivity is -170 Amps / Watt (A / W).

[0025] The 6 GHz signal generator was turned on at a frequency of 30MHz and an RF power of -22 dBm. and polarization and MZM DC bias were adjusted to optimize the resulting peak at 30MHz on the spectrum analyzer. The peak arising from optical excitation at 30 MHz was measured to be around -55dBm. The diplexer attenuates the signal measured by the spectrum analyzer from optical excitation by 13 dB at this frequency, so the actual signal level would be around -42 dBm.

[0026] The 2GHz signal generator w as turned on at a frequency of 40MHz and an RF power of 0 dBm, and two peaks appeared on the spectrum analyzer display: one at 40 MHz, - 30 dBm (which is the LO leaking through the diplexer to the spectrum analyzer), and one at 10 MHz, -55 dBm (which is the down-converted frequency of the 40MHz electrical fluctuation and 30MHz optical fluctuation). As both signal generators are producing near- pure sine w aves, the spectral shape of their signals should look like a spike at a certain frequency. The convolution of two spikes is a slightly broader spike- which means that the down-converted signal should look like a spike centered on the frequency resulting from subtracting the two input signal frequencies. Fig. 2 depicts the convolution of two signal spikes in the form of a broader spike in accordance with an illustrative embodiment.

[0027] If the 6GHz signal generator is turned off at this point, the peaks at 30 MHz and 10 MHz disappear, but the 40MHz peak from the electrical bias remains. This ensures that the 10MHz peak was not present as a result of some spurious signal generator output. When both signal generators are on, independently increasing their magnitude will cause a corresponding increase in magnitude on the “mixed” 10 MHz peak. Increasing the MZM RF input power to -19 dBm increases the 10 MHz peak from -55dBm to -52 dBm. Decreasing the LO RF power to -7 dBm decreases the 10 MHz peak from -55dBm to -62 dBm

[0028] The inventors then adjusted the frequency of the 6GHz signal generator to 39MHz. The optical peak at 39MHz has a measured height of -70 dBm on the spectrum analyzer. The diplexer attenuates the optical signal going to the spectrum analyzer by - 25.4dBm, which means the actual optical signal level is -45 dBm, similar to what was measured at 30 MHz.

[0029] Also, the peak at 10MHz disappeared. A peak appeared at 1MHz, which is to be expected, as the difference between the LO and optical input frequencies is 40 MHz - 39 MHz = 1 MHz. The measured power of this peak is -56 dBm, which again is similar to the previous results where the optical excitation was at a similar magnitude but different frequency. Fig. 3 depicts measured power of the peak in accordance with an illustrative embodiment.

[0030] Another experiment was conducted using a different phototransistor, without any Diplexer present (this would be functionally equivalent to shorting the diplexer's LP, HP, and RF ports together on the block diagram). The procedure for this experiment was as follows. The illuminator and camera were turned on in order to image the phototransistor chip and find the candidate phototransistor device. The laser was turned on, and the location of the back-reflected laser light on the camera image plane was noted. The laser beam was roughly aligned by using the IR objective’s kinematic mount to make the approximate location of the beam on the camera image plane collinear with the optically active area of the phototransistor.

[0031] Final alignment of the phototransistor was conducted by DC biasing the phototransistor to 1.4V and using the current readout of the 6.5 digit DMM to optimize photoresponse. Additionally, the IR objective kinematics were adjusted, collimator tip / tilt was adjusted, fiber attenuation was adjusted, MZM DC bias was adjusted, and polarization was adjusted

[0032] A DC photocurrent of ~10 mA was attained in this case. The DC photocurrent of the fiber-coupled PIN was measured with the handheld multimeter and monitored continuously throughout the experiment to track any optical power drift. Based on prior calibrations, the fiber-coupled PIN always receives approximately 3.5dB stronger signal than the device being coupled to from the free-space objective. Also, ~300uW of optical power was detected, meaning that ~150uW of optical power was incident on the phototransistor. This means the phototransistor responsivity is ~70 AAV.

[0033] The 6 GHz signal generator was turned on at a frequency of 12MHz and an RF power of -22 dBm, and polarization and MZM DC bias were adjusted to optimize the resulting peak at 30MHz on the spectrum analyzer. The peak arising from optical excitation at 30 MHz was measured to be around -42 decibel milliwatts (dBm). The 2GHz signal generator was turned on at a frequency of 9MHz and an RF power of -25 dBm. and three peaks appeared on the spectrum analyzer display: one at 9 MHz, -25 dBm (which is what the LO w as set to, as there is no diplexer shielding the spectrum analyzer), one at 3 MHz, -75 dBm (w hich is the up-converted frequency of the 12 MHz optical fluctuation and 9 MHz electrical fluctuation), and one at 21 MHz, -83 dBm (which is the up-converted frequency of the 12 MHz optical fluctuation and 9 MHz electrical fluctuation).

[0034] As both signal generators are producing near-pure sine waves, the spectral shape of their signals should look like a spike at a certain frequency. The convolution of two spikes is a slightly broader spike- which means that the down-converted signal should look like a spike centered on the frequency resulting from subtracting the tw o input signal frequencies, which it did. The inventors then wanted to compare the device optical response vs frequency to the signal-mixing response vs frequency. To do this, the LO was turned off, and the optical signal was swept from 2 MHz to 80 MHz, to find the device responsivity as a function of frequency. The inventors then changed the optical signal to 10MHz, and turned the LO on at 8 MHz. This produced a dow n-converted peak at 2MHz, and the power of this peak was measured.

[0035] The inventors also sw ept the optical signal from 10 MHz to 80 MHz and the LO signal from 8 MHz to 78 MHz, maintaining a 2 MHz separation the whole time, which produced a 2 MHz down-conversion peak at each set of frequencies. The inventors measured the magnitude of this 2MHz peak as the frequencies used to generate it w ere adjusted. As onemight expect, it decreased by almost exactly the same amount as the standard frequencyresponse peak does when the optical signal was swept to higher frequency values

[0036] The 2 MHz peak magnitude is proportional to the magnitudes of the two signals used to generate it. so if the optical response of the device decreases by some amount at higher frequencies, the down-converted peak resulting from an optical signal at that frequency should decrease by at least that much. Fig. 4 is a graph depicting optical fluctuation versus optical power in accordance with an illustrative embodiment.

[0037] Overall, these experimental results show concrete evidence that phototransistors can be used to perform frequency mixing / signal multiplication operations by exploiting the proportionality of phototransistor output current to both voltage bias and incident optical power. The inventors have therefore envisioned systems in which the signal multiplication feature is put into use. For example, the proposed signal multiplication can be used in systems in which measurements are taken at different depths of an object / sample. As known to those of skill in the art, the depth that can be measured is correlated to the frequency of the signal used to conduct the measurement. The correlation is 1: 1 such that each signal frequency corresponds to a specific depth. Thus, by' combining signals as described herein, higher frequency signals can be generated to measure greater depths within a sample without the use of costly high frequency components.

[0038] Fig. 5A displays the working principle behind the multiplication capability of electron injection photodetector / phototransistor devices in accordance with an illustrative embodiment. In an illustrative embodiment, the electron injection photodetector utilizes a type II heterojunction. For example, in one embodiment, the photodetector has a semiconductor heterostructure that includes a photon absorber having a layer of photon absorbing material which absorbs photons to generate electron-hole pairs. The semiconductor heterostructure also includes a earner injector adapted to inject carriers of a first type into the photon absorber. The carrier injector includes a layer of carrier injecting material capable of generating the carriers of the first type upon the application of a bias voltage. The carrier injector also includes a carrier trap disposed between the layer of carrier injecting material and the layer of photon absorbing material. The carrier trap also has a layer of potential barrier-forming material which forms a potential trap for carriers of a second ty pe. In an illustrative embodiment, the photon absorbing material, the carrier injecting material, and the potential barrier-forming material have a band structure with a type II band alignment.

[0039] As shown in Fig. 5 A, inputs to the electron injection phototransistor include a bias voltage Vb from a bias signal and optical power Popt from an optical signal, and an output is the phototransistor device current IEI. Increasing the bias voltage beyond intermediate values will bring the device out of a saturation region, and into a region where its responsivity is now a roughly linear function of bias voltage. As used herein, responsivity refers to the ratio of the device current to the optical power incident on the device. Fig. 5B is a graph depicting responsivity (y-axis) versus the bias voltage (x-axis) applied to the electron injection photodetector device in accordance with an illustrative embodiment.

[0040] Fig. 5C depicts a layer structure of an electron injection photodetector including the layer material, the layer thickness, and the dopant ty pe and concentration for each layer in accordance with an illustrative embodiment. As shown, the photodetector includes: Ino.53Gao.47As as a growth substrate (layer 1), Ino.53Gao.47As as the photon absorber (layer 2); GaAso.52Sbo.48 as the potential barrier-forming layer (layer 3); lno.52Alo.48As as layer of passivating material (layer 4); InP as a carrier injecting material (layer 5); and doped Ino.53Gao.47 As as an electrically conductive contact (layer 6). The layer structure of Fig. 5C is included as an example. In alternative embodiments, different layers, materials, and / or configurations may be used to form the electron injection photodetector.

[0041] Referring again to Fig. 5A. since the output current of the device is simply the product of its absorbed optical power and its responsivity, the output current of the device will then change proportionally to the product of any fluctuations of bias voltage and absorbed optical pow er. This principle can be used to multiply arbitrary' signals, which permits the execution of operations such as frequency mixing / downshifting within the electron injection device. The relationship of the inputs / output of the phototransistor are as follows:

[0042] Fig. 6 depicts an OCT system that incorporates an electron injection phototransistor to facilitate signal multiplication in accordance with an illustrative embodiment. This figure displays an example of an application (Sw ept-Source Optical Coherence Tomography, or SS-OCT) that could benefit from the El's frequency mixing as described previously. One benefit this frequency mixing capability offers to the SS-OCT application is the extension of the depth range of the measurement (w ith other variables held constant). This means that for a given DAQ, more depth information can be gathered, andwhen designing a system for a desired depth range, it would be possible to use a less expensive slower DAQ and all related electronics. In the example of Fig. 6, the local oscillator frequency could be set / swept such that it downshifts the SS-OCT signal (likely in bands that have similar width to the DAQ bandwidth), enabling the measurement of reflections producing higher frequencies (corresponding to interfaces deeper in the sample being measured) than the DAQ itself is natively able to measure. In alternative embodiments, instead of an OCT system, the signal multiplication described herein can be performed on a different type of system, such as a LIDAR system, a frequency modulated continuous wave (FMCW) system, etc.

[0043] In an illustrative embodiment, any of the operations or calculations described herein can be performed by a computing device that includes a processor, a memory, a user interface, a transceiver (receiver and / or transmitter), etc. The operations can be stored as computer-readable instructions in the memory. Upon execution by the processor of the computer-readable instructions, the computing device performs the operations, calculations, etc. described herein. As an example, Fig. 7 depicts a computing device 700 for performing signal multiplication in accordance with an illustrative embodiment. In an illustrative embodiment, the computing device 700 can be incorporated into a system such as an OCT system, a LIDAR system, a FMCW system, etc. Alternatively, one or more portions of the computing device 700 can be separate from the remaining components of the device, but in communication therewith through a network 735 and / or through a direct wired connection.

[0044] The computing device 700 includes a processor 705, an operating system 710, a memory 715. an input / output (I / O) system 720, a network interface 725, and a signal multiplication application 730. In alternative embodiments, the computing device 700 may include fewer, additional, and / or different components. The components of the computing device 700 communicate with one another via one or more buses or any other interconnect device.

[0045] The processor 705 of the computing device 700 can be in electrical communication with and used to control any of the device components described herein, such as the electrical signal generators, phototransistor, optical sources, etc. The processor 705 can be any type of computer processor known in the art, and can include a plurality’ of processors and / or a plurality of processing cores. The processor 705 can include a controller, a microcontroller, an audio processor, a graphics processing unit, a hardware accelerator, adigital signal processor, etc. Additionally, the processor 705 may be implemented as a complex instruction set computer processor, a reduced instruction set computer processor, an x86 instruction set computer processor, etc. The processor 705 is used to run the operating system 710, which can be a custom operating system specific to the requirements of the proposed system.

[0046] The operating system 710 is stored in the memory 715, which is also used to store programs, device data, algorithms, network and communications data, peripheral component data, and other operating instructions. The memory 715 can be one or more memory devices that include various types of computer memory such as flash memory, random access memory (RAM), dynamic (RAM), static (RAM), a universal serial bus (USB) drive, an optical disk drive, a tape drive, an internal storage device, a non-volatile storage device, a hard disk drive (HDD), a volatile storage device, etc.

[0047] The I / O system 720, or user interface, is the framework which enables users (and peripheral devices) to interact with the computing device 700. The I / O device 720 can include one or more keys or a keyboard, one or more buttons, one or more displays, a speaker, a microphone, etc. that allow the user to interact with and control the computing device 700 and other devices connected thereto. The I / O device 720 also includes circuitry and a bus structure to interface with peripheral computing components such as a battery or other power source, etc.

[0048] The network interface 725 includes transceiver circuitry that allows the computing device 700 to transmit and receive data to / from other devices such as user device(s), remote computing devices, seivers, websites, etc. The network interface 725 enables communication through the network 735, which can be one or more communication networks. The network 735 can include a cable network, a fiber network, a cellular network, a wi-fi network, a landline telephone network, a microwave network, a satellite network, etc. The network interface 725 also includes circuitry to allow device-to-device communication such as near field communication (NFC), Bluetooth® communication, etc.

[0049] The signal multiplication application 730 can include software and algorithms (e.g., in the form of computer-readable instructions) which, upon activation or execution by the processor 705, performs any of the various operations described herein such as controlling an electrical signal source, controlling an optical signal source, receivingmeasurements / data, processing received measurements / data, etc. The signal multiplication application 730 can utilize the processor 705 and / or the memory 715 as discussed above.

[0050] The word "illustrative" is used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as "illustrative" is not necessarily to be construed as preferred or advantageous over other aspects or designs. Further, for the purposes of this disclosure and unless otherwise specified, "a" or "an" means "one or more.”

[0051] The foregoing description of illustrative embodiments of the invention has been presented for purposes of illustration and of description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and as practical applications of the invention to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.

Claims

WHAT IS CLAIMED IS:1 . A signal multiplication device, the device comprising: a signal source that generates a bias voltage; a signal source that generates an optical signal; and an electron injection photodetector that receives the bias voltage and the optical signal, wherein the photodetector multiplies the bias voltage and the optical signal to generate an output photocurrent that is dependent on the bias voltage.

2. The device of claim 1, wherein the signal source that generates the optical signal comprises a laser of a light detection and ranging system.

3. The device of claim 1, further comprising a local oscillator, wherein a frequency of the local oscillator is set to downshift optical signal frequency.

4. The device of claim 1, wherein the electron injection photodetector performs frequency shifting based on the multiplication of the bias voltage and the optical signal.

5. The device of claim 1, wherein the output photocurrent corresponds to a signal having a first frequency that is able to take a first depth measurement of a sample.

6. The device of claim 5, wherein the optical signal has a second frequency that is less than the first frequency.

7. The device of claim 1, wherein the bias voltage is set to take the electron injection photodetector out of a saturation region, and into a region where responsivity of the electron injection photodetector is a linear function of bias voltage.

8. The device of claim 7, wherein the output photocurrent comprises a product of absorbed pow er from the optical signal and the responsivity of the electron injection photodetector.

9. The device of claim 1, wherein the electron injection photodetector includes a type II heterojunction.

10. The device of claim 1, wherein the signal source that generates the optical signal comprises a swept-source laser of an optical coherence tomography system.

11. The device of claim 10, further comprising a bias tee that is configured to receive and distribute the bias voltage.

12. The device of claim 11, further comprising a diplexer that is electrically connected to both the bias tee and the electron injection photodetector.

13. A method for performing signal multiplication, the method comprising: receiving, by an electron injection photodetector, a bias voltage signal; receiving, by the electron injection photodetector, an optical signal; and multiplying, by the electron injection photodetector, the bias voltage signal and the optical signal to generate an output photocurrent that is dependent on the bias voltage signal.

14. The method of claim 13, further comprising generating the optical signal by a swept- source laser of an optical coherence tomography system.

15. The method of claim 13, wherein the electron injection photodetector includes a type II heterojunction.

16. The method of claim 13, further comprising setting a frequency of a local oscillator, wherein the frequency of the local oscillator is set to downshift the optical signal.

17. The method of claim 13, further comprising performing, by the electron injection photodetector, frequency shifting based on the multiplication.

18. The method of claim 13, wherein the output photocunent corresponds to a signal having a first frequency that is able to take a first depth measurement of a sample, and wherein the optical signal has a second frequency that is less than the first frequency.

19. The method of claim 13, further comprising receiving and distributing the bias voltage signal by a bias tee.

20. The method of claim 19, further comprising a diplexer that is electrically connected to both the bias tee and the photodetector.

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