WO2014088669A2

WO2014088669A2 - Systems and methods for scalable readouts for photon detectors using integrated modulators and wavelength-division multiplexing

Info

Publication number: WO2014088669A2
Application number: PCT/US2013/059621
Authority: WO
Inventors: Dirk R. ENGLUND; Xiaolong Hu; Karl K. Berggren
Original assignee: The Trustees Of Columbia University In The City Of New York
Priority date: 2012-09-14
Filing date: 2013-09-13
Publication date: 2014-06-12
Also published as: WO2014088669A3

Abstract

Techniques for scalable readouts for photon detectors using integrated modulators and wavelength-division multiplexing are disclosed herein. An exemplary apparatus can include a plurality of photon detectors, each adapted to provide an output signal in response to detected photons. A plurality of modulators each can be coupled to a respective one of the plurality of photon detectors. The modulators can convert an output of the photon detectors into an optical signal. A wavelength-division multiplexing (WDM) device can be coupled to each of the plurality of modulators and can guide the optical signal from each modulator into one or more outputs.

Description

SYSTEMS AND METHODS FOR SCALABLE READOUTS FOR PHOTON DETECTORS USING INTEGRATED MODULATORS AND WAVELENGTH- DIVISION MULTIPLEXING PATENT APPLICATION SPECIFICATION

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR

DEVELOPMENT

This invention was made with government support under Grant No. W91 lNF-10 - 1 -0416, awarded by the Army Research office/DARPA. The

government has certain rights in the invention.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. Provisional Application Serial No. 61/701,333, filed September 14, 2012, the disclosure of which is incorporated by reference herein.

BACKGROUND

The disclosed subject matter relates to systems and methods for scalable readouts for photon detectors using integrated modulators and wavelength- division multiplexing.

Superconducting-nanowire single-photon detectors (SNSPDs, or, SSPDs) can be used for photon-counting technology and are well-suited for infrared wavelengths. For example, SNSPDs have been applied to imaging, detection of low optical signals, lidar, integrated quantum optics, integration of antennae and waveguides with the nanowires, and long-distance quantum key distribution. One challenge, however, can be to scaling from one or a few SNSPDs to a large array of SNSPDs, for example for imaging applications such as infrared imaging. It can be difficult to scale from one or a few SNSPDs to an array of SNSPDs because the readout circuit associated with the SNSPDs can lack scalability. For example, one detector can use a set of bias and readout circuits. However, these circuits can cause heating at the location of the SNSPD in the cryostat and therefore can make thermal management (i.e., cooling down the SNSPDs) impractical, particularly at cryogenic temperatures at which the SNSPDs operate. There is a need for an improved technique for scalable readouts for photon detectors.

SUMMARY

Systems and methods for scalable readouts for photon detectors using integrated modulators and wavelength-division multiplexing are disclosed herein.

In one aspect of the disclosed subject matter, techniques for detecting photons are disclosed. An exemplary apparatus can include a plurality of photon detectors, each adapted to provide an output signal in response to detected photons. A plurality of modulators each can be coupled to a respective one of the plurality of photon detectors. The modulators can convert an output of the photon detectors into an optical signal. A wavelength-division multiplexing (WDM) device can be coupled to each of the plurality of modulators. The WDM device can guide the optical signal from each modulator into one or more outputs.

In some embodiments, the plurality of photon detectors can be superconducting-nanowire single-photon detectors (SNSPDs). For example, at least one of the plurality of SNSPDs can be a niobium nitride (NbN) nanowire.

Additionally or alternatively, each of the plurality of SNSPDs can have a bias current.

In some embodiments, the output of each of the plurality of photon detectors can be an output voltage. Additionally or alternatively, a plurality of amplifiers each can be coupled to a respective one of the plurality of photon detectors and adapted to amplify the output thereof. For example, each amplifier can amplify the output of the respective one of the plurality of photon detectors up to one volt.

In some embodiments, a continuous wave light source can be coupled to at least one of the plurality of modulators. The modulator(s) can modulate light from the continuous wave light source in response to the output of the respective one of the plurality of photon detectors to thereby convert the output into the optical signal.

In some embodiment, at least one of the modulators can be a silicon ring resonator. Additionally or alternatively, the plurality of photon detectors can operate at a first temperature of less than 4 K and the silicon ring resonator(s) can operate at a second temperature of greater than 100 K.

In some embodiments, each optical signal can have a respective wavelength that is different than a respective wavelength of each other optical signal. Additionally or alternatively, a plurality of waveguides each can be coupled to a respective one of the plurality of modulators. The waveguides can guide the optical signal from the respective one of the modulators to the WDM device. In some embodiments, the one or more outputs can be at least one optical fiber. For example, optical fiber can be a single-mode optical fiber. Alternatively, a plurality of single- mode optical fibers can be used.

In some embodiments, a demultiplexing device can be coupled to the one or more outputs. The demultiplexing device can demultiplex the optical signals. Additionally or alternatively, a readout circuit can be coupled to the demultiplexing device to process the demultiplexed optical signals.

In another aspect of the disclosed subject matter, methods for detecting photons are disclosed. An exemplary method can include detecting photons to generate one or more outputs corresponding to the detected photons. The one or more outputs can be modulated into one or more optical signals. The optical signals can be guided by wavelength-division multiplexing into at least one optical fiber.

In some embodiments, the one or more outputs can be one or more output voltages. Additionally or alternatively, the outputs can be amplified.

In some embodiments, the one or more outputs can be modulated into one or more optical signals, each having a unique wavelength. Additionally or alternatively, the one or more optical signals can be demultiplexed. Additionally or alternatively, the demultiplexed optical signals can be processed.

In another aspect of the disclosed subject matter, methods of making an apparatus for detecting photons are disclosed. An exemplary method can include providing a plurality of photon detectors. A plurality of modulators can be coupled to the plurality of photon detectors such that each modulator is coupled to a respective one of the plurality of photon detectors. The modulators can convert an output of the respective one of the plurality of photon detectors into an optical signal. A

wavelength-division multiplexing device can be coupled to the plurality of modulators. The WDM device can guide the optical signal from each modulator. At least one optical fiber can be coupled to the WDM device. The optical fiber can receive the guided optical signals.

In some embodiments, the plurality of photon detectors can be a plurality of SNSPDs. For example, at least one of the plurality of SNSPDs includes a niobium nitride (NbN) nanowire. Additionally or alternatively, at least one current source can be coupled to the plurality of SNSPDs. The current source can provide a bias current to each of the plurality of SNSPDs. Additionally or alternatively, a plurality of amplifiers can be coupled to the plurality of photon detectors. Each amplifier can be coupled to a respective one of the plurality of photon detectors. Each amplifier can amplify the output of the respective one of the plurality of photon detectors.

In some embodiments, a continuous wave light source can be coupled to at least one of the plurality of modulators. The modulator(s) can modulate light from the continuous wave light source in response to the output of the respective one of the plurality of photon detectors to convert the output into the optical signal. For example, at least one of the modulators can be a silicon ring resonator. Additionally or alternatively, a plurality of waveguides can be coupled to the plurality of modulators such that each waveguide is coupled to a respective one of the plurality of modulators. The waveguides can guide the optical signal from the respective one of the plurality of modulators to the WDM device.

In some embodiments, the optical fiber can be one or more a single- mode optical fibers. Additionally or alternatively, a demultiplexing device can be coupled to the optical fiber(s). The demultiplexing device can demultiplex the optical signals. Additionally or alternatively, a readout circuit can be coupled to the demultiplexing device to process the demultiplexed optical signals.

The accompanying drawings, which are incorporated and constitute part of this disclosure, illustrate certain embodiments of the disclosed subject matter and serve to explain the principles of the disclosed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an exemplary apparatus for detecting photons, in accordance with some embodiments of the disclosed subject matter.

FIG. 2 is a diagram illustrating an exemplary method for detecting photons, in accordance with some embodiments of the disclosed subj ect matter.

FIG. 3 is a diagram illustrating an exemplary method for making an apparatus for detecting photons, in accordance with some embodiments of the disclosed subject matter. Throughout the drawings, similar reference numerals and characters, unless otherwise stated, are used to denote like features, elements, components or portions of the illustrated embodiments. Moreover, while the present disclosed subject matter will now be described in detail with reference to the FIGS., it is done so in connection with the illustrative embodiments.

DETAILED DESCRIPTION

Techniques for scalable readouts for photon detectors using integrated modulators and wavelength-division multiplexing are presented. As further discussed below in connection with the illustrative embodiments, an apparatus for detecting photons can include a plurality of photon detectors. For purpose of illustration and not limitation, the photon detectors can be superconducting-nanowire single-photon detectors (SNSPDs, or, SSPDs), such as for example those disclosed in U.S. Patent Publication No. 2012/0077680, which is hereby incorporated by reference herein in its entirety. The SNSPDs can be used at low temperatures (e.g. below 4 K for detectors based on niobium nitride (NbN) or below 2 K for detectors based on tungsten silicide (WSi₂)). At such temperatures, the SNSPDs can be in a

superconducting state in which they can be modeled as inductors, and a bias current can be applied to the SNSPDs.

One or more incident photons can be absorbed by an SNSPD, and when an incident photon is absorbed by an SNSPD with a bias current slightly below the critical current of the superconducting nanowire, a resistive region called hot-spot can be generated, which can generate an output, e.g. a voltage pulse or a current pulse. Electrically connecting a readout circuit, e.g. by SubMiniature version A (SMA) cables, to each photon detector can generate heat at each such connection. As such, it can be difficult to cool down the SNSPDs to sufficiently low temperatures. Instead, a modulator can be coupled to each photon detector, and the modulator can be adapted to convert the output voltage pulse into an optical signal. The optical signals can be guided by wavelength division multiplexing into one or more optical fibers. Thus, instead of a multitude of thermally conductive metal cables, one or a few thermally insulating optical fibers can be used.

Referring to FIG. 1, an exemplary apparatus 100 for detecting photons is depicted. A plurality of photon detectors 101 can be adapted to provide an output signal 110 in response to detected photons. For example, the plurality of photon detectors 101 can be a plurality of any suitable photon detectors, such as a plurality of SNSPDs or a plurality of Transition Edge Sensors (TES). For purpose of illustration and not limitation, the plurality of photon detectors 101 can be SNSPD 1 , SNSPD 2, ..., SNSPD n (collectively "SNSPDs") as depicted in FIG. 1. While the illustrative embodiments will be discussed with reference to the SNSPDs depicted in FIG. 1 , the techniques described herein can be applied to any suitable photon detectors 101 or combination of photon detectors 101, including those discussed above.

The SNSPDs 101 can be a plurality of nanowires formed of any suitable superconducting material. For example, the SNSPDS 101 can be nanowires formed of niobium nitride (NbN), tungsten silicide (WSi₂), niobium silicide, tantalum nitride, or other suitable superconducting materials. For example, the SNSPDs can be a plurality of NbN nanowires, which can be modeled as an inductor of about 10- 1000 nH in its superconducting state and a resistor R_k of about 1 ΜΩ or higher in its normal state.

Each of the plurality of SNSPDs 101 can have a bias current Iyas applied thereto. The bias current Ibias can be applied by any suitable current source 120. For purpose of illustration and not limitation, the bias current l^_ms can be less than the critical current of the SNSPDs. For example, the bias current Ibias can be tens of micro amps (μΑ). When an incident photon is absorbed by an SNSPD 101 with a bias current slightly below (e.g., 5-20% below) the critical current of the

superconducting nanowire, a resistive region called hot-spot can be generated, which can cause an avalanche breakdown that generates an output 110, e.g. a voltage pulse or a current pulse.

For example, the output 110 of each of the SNSPDs can be an output voltage VI, V2, ..., Vn. Thus, when an SNSPD 101 absorbs an incident photon, the bias current I_bias flows through the capacitor 141. For purpose of illustration and not limitation, the output voltages 110 can be equal to the bias current Ibias multiplied by the input impedance of the capacitor 141. For example, the bias current Ibias can be tens of micro amps (μΑ), the input impedance of the capacitor 141 can be about 50 ± 2 Ω.

A plurality of modulators 130, for example Ml , M2, , .. Mn, can each be coupled to a respective one of the plurality of SNSPDs 101. The modulators 130 can be adapted to convert the outputs 1 10 of the SNSPDs 101 into optical signals 150, for example λι, λ₂, . λ_η. For purpose of illustration and not limitation, at least one of the plurality of modulators 130 can be a silicon ring resonator. The outputs 110 can drive the silicon ring modulators 130, which can modulate the optical signals 150 in silicon waveguides. In some embodiments, on-chip amplifiers 142 can be used to amplify the outputs 1 10. For example, if the outputs 110 are not sufficient to drive the modulators 130, the amplifiers 142 can amplify the outputs 1 10 to a suitable magnitude. For purpose of illustration and not limitation, silicon ring modulators 130 can be driven by a voltage of 0. IV - 5 V, depending on the design. Alternatively, the modulators 130 can be Mach-Zehnder modulators, which can be operated in near- extinction in one of the two output arms. In such a case, the phases on the two arms of the Mach-Zehnder can be set so that the fields interfere destructively on the near- extinction output mode, and constructively on the other. For example, in the near- extinction mode, even a small phase change on one of the arms of the Mach-Zehnder modulator can result in a strong change of the optical signal in the output arm. Thus, even a small voltage from the SNSPD 101, e.g. below 10 mV with little or no amplification, can translate the SNSPD signal onto the near-extinction output.

Amplifiers 142 can be used to amplify the outputs 1 10 from less than 1 V to about 1 V. For example, the amplifiers 142 can be any suitable amplifiers, such as superconducting amplifiers, superconducting switching amplifiers, a high electron mobility transistor (HEMT) amplifier, or a complementary metal-oxide- semiconductor (CMOS) amplifier.

For purpose of illustration and not limitation, each optical signal 150 can have a respective wavelength different than a respective wavelength of each other optical signal 150. For example, a first optical signal 150 can have a wavelength λι that is different from a second optical signal 150 having a wavelength λ₂ and different than any other optical signal 150 having a wavelength λ_η. In some embodiments, a continuous wave (CW) light source can be coupled to at least one of the plurality of modulators 130. For example, a CW light source of a distinct wavelength can be coupled to each of the modulators 130. The modulators 130 can be adapted to modulate light from their respective CW light source in response to the outputs 1 10 of the respective one of the plurality of SNSPDs 101 to thereby convert the outputs 110 into the optical signals 150.

A wavelength-division multiplexing (WDM) device 160 can be coupled to each of the plurality of modulators 130. The WDM device can be adapted to guide the optical signal 150 from each modulator 130 into one or more outputs 170. For example, the WDM device 160 can be any suitable WDM device 160, such as a dense wavelength division multiplexing DWDM device as depicted in FIG. 1. In some embodiments, the WDM device 160 can be coupled to the modulators 130 by a plurality of waveguides 171. For example, each waveguide 171 can be coupled to a respective one of the plurality of modulators 130 and adapted to guide the optical signal 150 from the respective one of the plurality of modulators 130 to the WDM device 160. Thus, the WDM technology 160 can be used to guide the modulated optical signals 150 into one or more outputs 170. The output 170 can be any suitable output for the guided optical signals 150. For purpose of illustration and not limitation, the outputs 170 can be at least one optical fiber. For example, the output 170 can be one or more single-mode optical fibers. As such, electrical connections between the SNSPDs 101 and the output 170 are not necessary. For example, WDM technology can allow one single-mode fiber 170 to guide all the output optical signals 150.

In some embodiments, a demultiplexing device can be coupled to the at least one output 170, e.g. coupled to the single-mode optical fiber, and adapted to demultiplex the optical signals 150. A readout circuit can be coupled to the demultiplexing device to process the demultiplexed optical signals 150. For example, the optical signals 150 in an output single-mode fiber 170 can then be de -multiplexed and processed at room temperature.

The performance of the modulators 130 can be dependent on the temperature. Temperatures at which different components operate can be carefully designed and managed. For example, a modulator 130, such as a silicon ring resonator, can function based on free-carrier plasma dispersion effect. The refractive index of the silicon (n = 3.5), and thus the resonance of the ring and modulation depth, can be changed by changing the concentration of electrons and holes. Free- carrier plasma dispersion effect can be temperature-dependent. In particular, at cryogenic temperatures, carriers can be frozen and carrier density change due to injection can be small or slow. To address temperature dependence, in some embodiments, the apparatus can be designed with two or more regions with different temperatures and weak thermal link in between. For example, as depicted in FIG. 1, a first stage 180 can include the SNSPDs 101 and can be at very low temperature, e.g. less than 4 K, to cool down the SNSPDs 101. On the same or a different chip, a second stage 190 can include the modulators 130 and can be above 50 K, at which temperature carrier freeze-out is not significant, permitting effective carrier injection to the silicon modulators. Thus, the SNSPDs 101 can operate at a first temperature, e.g. 4 K, while the modulators 130 can operate at a second temperature, e.g. 50-100 K. In some embodiments, the electrical connections between the SNSPD 101 chip at 4 K in the first stage 180 and the modulators at 50-100K in the second stage 190 can be short and thin so that the heating of the lower-temperature first stage 180 from the higher-temperature second stage 190 can be much lower than if the lower-temperature first stage 180 were connected to a room temperature stage outside the cryostat.

Referring to FIG. 2, an exemplary method for detecting photons is depicted. An exemplary method can include detecting photons to generate one or more outputs corresponding to the detected photons (201), for example as described above, in some embodiments, the outputs can be one or more output voltages. The outputs can be modulated into one or more optical signals (203), for example as described above. In some embodiments, the outputs can be modulated into one or more optical signals, each having a unique wavelength. The optical signals can be guided by wavelength-division multiplexing into at least one optical fiber (204), for example as described above.

In some embodiments, the method can include amplifying each of the one or more outputs (202), for example as described above. Additionally or alternatively, the method can include demultiplexing the one or more optical signals (205), for example as described above. Additionally or alternatively, the

demultiplexed optical signals can be processed (206) for example as described above.

Referring to FIG. 3, an exemplary method of making an apparatus for detecting photons is disclosed. The method can include providing a plurality of photon detectors (301). For example, the photon detectors can be provides in a cryostat or some other environment having the temperatures described above. The photon detectors can be coupled to a plurality of modulators such that each modulator is coupled to a respective one of the plurality of photon detectors, and the modulators can convert an output of the respective one of the plurality of photon detectors into an optical signal (302), for example as described above. A wavelength-division multiplexing device can be coupled to the plurality of modulators, and the WDM device can guide the optical signal from each modulator (303), for example as described above. At least one optical fiber can be coupled to the WDM device, and the optical fiber can receive the guided optical signals (304), for example as described above.

In some embodiments, the plurality of photon detectors can be a plurality of SNSPDs. For example, at least one of the plurality of SNSPDs can be a niobium nitride (NbN) nanowire. Additionally or alternatively, at least one current source can be coupled to the plurality of SNSPDs, and the current source can provide a bias current to each of the plurality of SNSPDs. In some embodiments, a plurality of amplifiers can be coupled to the plurality of photon detectors such that each amplifier is coupled to a respective one of the plurality of photon detectors, and the amplifiers can amplify the output of the respective one of the plurality of photon detectors (306), for example as described above.

In some embodiments, a continuous wave light source can be coupled to at least one of the plurality of modulators, and the modulators can modulate light from the continuous wave light source in response to the output of the respective one of the plurality of photon detectors to thereby convert the output into the optical signal (307), as described above. Additionally or alternatively, at least one of the

modulators can be a silicon ring resonator. Additionally or alternatively, a plurality of waveguides can be coupled to the plurality of modulators such that each waveguide is coupled to a respective one of the plurality of modulators, and the waveguides can guide the optical signal from the respective one of the plurality of modulators to the WDM device (308), as described above.

In some embodiments, the optical fiber can be a single-mode optical fiber. Additionally or alternatively, a plurality of single-mode optical fibers can be used.

In some embodiments, a demultiplexing device can be coupled to the optical fiber, and the demultiplexing device can demultiplex the optical signals (309), as described above. Additionally or alternatively, a readout circuit can be coupled to the demultiplexing device to process the demultiplexed optical signals (310), as described above.

The disclosed subject matter can allow the construction of a cryostat containing many SNSPDs, e.g. hundreds or more SNSPDs, with one single-mode optical fiber as the output channel. Such an SNSPD system can, for example, enable infrared imaging at a single-photon level and can impact the fields of quantum optics and astronomy where infrared single-photon counting can be utilized. The foregoing merely illustrates the principles of the disclosed subject matter. Various modifications and alterations to the described embodiments will be apparent to those skilled in the art in view of the teachings herein. It will thus be appreciated that those skilled in the art will be able to devise numerous techniques which, although not explicitly described herein, embody the principles of the disclosed subject matter and are thus within its spirit and scope.

Claims

1. An apparatus for detecting photons, comprising:

a plurality of photon detectors, each adapted to provide an output signal in response to detected photons;

a plurality of modulators, each coupled to a respective one of the plurality of photon detectors and adapted to convert an output thereof into an optical signal; and

a wavelength-division multiplexing (WDM) device, coupled to each of the plurality of modulators, and adapted to guide the optical signal from each modulator into one or more outputs.

2. The apparatus of claim 1, the plurality of photon detectors comprising a plurality of superconducting-nanowire single-photon detectors (SNSPDs).

3. The apparatus of claim 2, wherein at least one of the plurality of SNSPDs comprises a niobium nitride (NbN) nanowire.

4. The apparatus of claim 2, each of the plurality of SNSPDs having a bias current.

5. The apparatus of claim 1, the output of each of the plurality of photon detectors comprising an output voltage.

6. The apparatus of claim 5, further comprising a plurality of amplifiers, each coupled to a respective one of the plurality of photon detectors and adapted to amplify the output thereof.

7. The apparatus of claim 6, wherein each amplifier comprises an amplifier adapted to amplify the output of the respective one of the plurality of photon detectors up to one volt.

8. The apparatus of claim 1, further comprising a continuous wave light source coupled to at least one of the plurality of modulators, the at least one of the plurality of modulators adapted to modulate light from the continuous wave light source in response to the output of the respective one of the plurality of photon detectors to thereby convert the output into the optical signal.

9. The apparatus of claim 1, at least one of the plurality of modulators comprising a silicon ring resonator.

10. The apparatus of claim 9, the plurality of photon detectors adapted to operate at a first temperature of less than 4 K and the silicon ring resonator adapted to operate at a second temperature of greater than 100 K.

11. The apparatus of claim 1, each optical signal having a respective wavelength different than a respective wavelength of each other optical signal.

12. The apparatus of claim 1, further comprising a plurality of waveguides, each waveguide coupled to a respective one of the plurality of modulators and adapted to guide the optical signal from the respective one of the plurality of modulators to the WDM device.

13. The apparatus of claim 1 , the one or more outputs comprising at least one optical fiber.

14. The apparatus of claim 13, the at least one optical fiber comprising a single-mode optical fiber.

15. The apparatus of claim 13, the at least one optical fiber comprising a plurality of single-mode optical fibers.

16. The apparatus of claim 1, further comprising a demultiplexing device coupled to the one or more outputs and adapted to demultiplex the optical signals.

17. The apparatus of claim 16, further comprising a readout circuit coupled to the demultiplexing device to process the demultiplexed optical signals.

18. A method for detecting photons, comprising:

detecting photons to generate one or more outputs corresponding to the detected photons;

modulating the one or more outputs into one or more optical signals; and

guiding the one or more optical signals by wavelength-division multiplexing into at least one optical fiber.

19. The method of claim 38, the one or more outputs comprising one or more output voltages.

20. The method of claim 19, further comprising amplifying each of the one or more outputs.

21. The method of claim 18, wherein the modulating further comprises modulating the one or more outputs into one or more optical signals, each having a unique wavelength.

22. The method of claim 18, further comprising demultiplexing the one or more optical signals.

23. The method of claim 22, further comprising processing the demultiplexed optical signals.

24. A method of making an apparatus for detecting photons, comprising: providing a plurality of photon detectors;

coupling a plurality of modulators to the plurality of photon detectors, each modulator coupled to a respective one of the plurality of photon detectors and adapted to convert an output of the respective one of the plurality of photon detectors into an optical signal; and

coupling a wavelength-division multiplexing device to the plurality of modulators, the WDM device adapted to guide the optical signal from each modulator; and

coupling at least one optical fiber to the wavelength-division multiplexing device, the at least one optical fiber adapted to receive the guided optical signals.

25. The method of claim 24, the plurality of photon detectors comprising a plurality of superconducting-nanowire single-photon detectors (SNSPDs).

26. The method of claim 25, further comprising coupling at least one current source to the plurality of SNSPDs, the at least one current source adapted to provide a bias current to each of the plurality of SNSPDs.

27. The method of claim 24, further comprising coupling a plurality of amplifiers to the plurality of photon detectors, each amplifier coupled to a respective one of the plurality of photon detectors and adapted to amplify the output of the respective one of the plurality of photon detectors.

28. The method of claim 24, further comprising coupling a continuous wave light source to at least one of the plurality of modulators, the at least one of the plurality of modulators adapted to modulate light from the continuous wave light source in response to the output of the respective one of the plurality of photon detectors to thereby convert the output into the optical signal.

29. The method of claim 24, further comprising coupling a plurality of waveguides to the plurality of modulators, each waveguide coupled to a respective one of the plurality of modulators and adapted to guide the optical signal from the respective one of the plurality of modulators to the WDM device.