Light injection synchronization device

Abstract

To reduce backward reflection by using only a phase retarder without requiring a Faraday isolator which is bulkier and more expensive.SOLUTION: There is provided a laser injection synchronization device 400, which comprises: a primary laser 402 configured to emit light of first polarization; a secondary laser 404 configured such that light from the primary laser is injected, the secondary laser being connected to the primary laser by an optical path; and a phase retarder 410 provided in the optical path between the primary laser and secondary laser. The phase retarder is configured to convert the light of first polarization into light of second polarizer. The first polarization is one of linear polarization and circular polarization, and the second polarization is the other.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] TECHNICAL FIELD The embodiments described herein generally relate to techniques for optical injection locking. [Background technology]

[0002] Optical injection locking is a technique widely used in laser systems, including in fields such as quantum communications. Optical injection locking typically involves a primary laser and a secondary laser. For example, light from the primary laser can be used to define the phase between pulses output by the secondary laser. [Brief explanation of the drawings]

[0003] [Figure 1] Figure 1 shows a Faraday isolator. [Figure 2A] FIG. 2A is a schematic diagram of an apparatus for optical injection locking. [Figures 2B-2D] 2B-2D illustrate the behavior of the device of FIG. 2A. [Figure 3A] FIG. 3A is a block diagram of an apparatus for optical injection locking. [Figure 3B] FIG. 3B illustrates the behavior of the device of FIG. 3A. [Figure 4] FIG. 4 shows an optical injection locking device according to an embodiment. [Figure 5] FIG. 5 shows a further optical injection locking device according to a further embodiment. [Figure 6] FIG. 6 shows a further optical injection locking device according to a further embodiment. [Figure 7] FIG. 7 is a flow chart illustrating a method for optical injection locking according to one embodiment. [Figure 8A] FIG. 8A shows a transmitter for a quantum key distribution (QKD) system. [Figure 8B] FIG. 8B shows a receiver for a quantum key distribution (QKD) system. DETAILED DESCRIPTION OF THE INVENTION

[0004] In one embodiment, a primary laser configured to emit light of a first polarization; a secondary laser configured to be injected with light from the primary laser, the secondary laser being connected to the primary laser by an optical path; a primary phase retarder disposed in the optical path between the primary laser and the secondary laser, the primary phase retarder configured to convert light of the first polarization to light of the second polarization; Equipped with An apparatus for injection locking a laser is provided, wherein the first polarization is one of linear and circular polarization and the second polarization is the other.

[0005] The above device allows for mitigation of the effects of back reflections, for example, by suppressing back reflection-related laser disturbances. In injection-locked systems, as in many other optical systems, it is desirable to prevent back reflections. For example, some of the light emitted by the primary laser may be reflected back at the input of the secondary laser. If this light re-enters the primary laser, it may interfere with the operation of the primary laser, which may, for example, disrupt the desired phase relationship between pulses.

[0006] Anti-reflective coatings may be used in such systems, however, it should be noted that the input of the secondary laser cannot be perfectly anti-reflective because the secondary laser must form a reflective cavity in order to operate as a laser, and therefore anti-reflective coatings cannot be completely effective in this case.

[0007] FIG. 1 illustrates the use of a Faraday isolator 100 to block back reflections. For example, such an isolator 100 can be inserted between the primary and secondary lasers of an optical injection locking system. The Faraday isolator 100 includes two polarizers 102 and 104 on either side of a Faraday rotator 106. Incident light 108 is polarized by the first polarizer 102 to have a particular linear polarization (e.g., horizontal polarization) and then passes through the Faraday rotator 106, where the polarization is rotated by 45°. The second polarizer 104 is oriented to transmit this diagonal light 110 out of the Faraday isolator 100. Back-reflected light 112 with the same diagonal polarization can re-enter the Faraday isolator 100 by passing through the second polarizer 104. Upon passing back through the Faraday rotator 106, the polarization of the back-reflected light is rotated by an additional 45°, resulting in vertical polarization. Because the first polarizer 102 is oriented to transmit horizontal light, vertically back-reflected light will be filtered out by the first polarizer 102 and cannot pass back out of the Faraday isolator 100. In this way, the Faraday isolator 100 prevents back-reflections from returning to any earlier optical components in the beam path.

[0008] Faraday isolator 100 relies on non-reciprocal rotation by a Faraday rotator 106. This is achieved using a strong permanent magnetic field B within Faraday rotator 106, as shown in FIG.

[0009] Note that due to the presence of polarizers 102 and 104, the Faraday isolator works regardless of the polarization of the incident light 108, 112 from either direction, since the light is initially polarized by the respective polarizers 102, 104.

[0010] 2A-2D and 3A-3B, we describe a particular type of optical source that enables the creation of an ultra-compact, high-performance quantum key distribution (QKD) transmitter. This optical source 200, shown in FIG. 2A, uses gain switching and optical injection locking to directly phase modulate the pulses, eliminating the need for an external phase modulator.

[0011] The light source 200 comprises a pulsed secondary laser 204 into which pulses from the primary laser 202 are injected to define the phase between the output pulses of the secondary laser based on optical injection locking. The primary laser 202 performs the task of phase preparation, while the secondary laser 204 performs the task of pulse generation. The description of Figures 2A-2D focuses more on the control of the primary laser. The description of Figures 3A and 3B focuses more on the secondary laser and the combination of the two lasers.

[0012] 2A, a primary laser diode 202 is connected to a secondary laser diode 204 via an optical circulator 206. It should be noted that the primary laser diode 202 and the secondary laser diode 204 may be identical, and the terms "primary" and "secondary" are used merely for clarity and do not imply any physical difference between the primary laser diode 202 and the secondary laser diode 204.

[0013] The primary laser 202 is used for phase preparation and is directly modulated to generate long pulses from quasi-steady-state emission. Each of these pulses coherently seeds a block of two or more secondary short optical pulses emitted by gain-switching the secondary or pulse-generating laser 204. The phase-ready laser 202 is biased to generate nanosecond or even shorter quasi-steady-state optical pulses with shallow intensity modulation, which also modify the optical phase. For clock rates greater than 1 GHz, the pulse width is less than 1 ns. The gain-switched pulse-generating laser 204 emits short optical pulses that inherit the optical phase prepared by the phase-ready laser. The duration of each phase-ready laser pulse can be varied to seed pulse trains of different lengths.

[0014] The relative phase between the secondary pulses depends on the phase evolution of the primary pulse and can be set to any value by directly modulating the drive current applied to the primary or phase-ready laser 202 .

[0015] For example, the relative phase φ between two secondary pulses can be obtained by introducing a small perturbation to the drive signal of the phase-ready laser 202. Similarly, the relative phases between three secondary pulses can be set to φ and φ by adding two small perturbations to the drive signal of the primary laser 202.

[0016] In principle, such perturbations in the drive signal would cause deleterious variations in the intensity and frequency of the primary pulses, but these can be avoided by switching off the gain of the secondary laser 204 in response to the perturbation signal. Effectively, the secondary laser 204 also acts as a filter to remove residual modulation.

[0017] To understand how the optical phase is set by perturbing the drive signal applied to a phase-ready laser, it is useful to consider an above-threshold continuous-wave (CW) laser emitting at a center frequency ν.

[0018] Figure 2B shows the duration t m 2C is a plot of the optical frequency of the phase-ready laser 202 under a small perturbation of .mu.m. FIG. 2D is a plot of the optical phase trajectory of the phase-ready laser 202 with and without perturbation.

[0019] When a small perturbation is applied to the drive signal, the optical frequency shifts by an amount Δν, changing the course of the phase evolution. When the perturbation is switched off, the frequency returns to its initial value ν. This perturbation will produce a phase difference of Δφ=2πΔνt m where t m is the duration of the perturbation. By optical injection, this phase difference is transferred onto the pair of secondary pulses emitted by the pulse generating laser 204, as shown in Figure 2D.

[0020] Here, the perturbation signal is a voltage modulation applied to the phase-prepared laser 202. The optical frequency change arises from the effect of carrier density on the refractive index in the laser-active medium within the primary laser diode 202. The confinement of the laser cavity allows the optical field to oscillate back and forth within the cavity and experience a refractive index change throughout the duration of the perturbation. Laser cavity enhancements, as described below, allow the phase modulation half-wave voltage to be kept below 1 V. This cavity feature is not present in conventional phase modulators, where light passes through the electro-optic medium only once, thus limiting the interaction distance to the device length.

[0021] Small changes to the electrical controller signal of the primary laser 202 (e.g., less than 1 volt, i.e., much less than required by a conventional lithium niobate phase modulator) can cause a transient change in the output frequency of the output of the primary laser 202, which in turn changes the output phase of the optical output of the secondary laser 204.

[0022] In this embodiment, the primary laser 202 is configured to output a sequence of optical pulses comprising a sequence of pairs. The phases of the pulses output by the primary laser 202 are controlled such that the phase between pulses in the same pair is randomly selected from one of a set of phase differences, and there is a random phase difference between pulses from different pairs. In one embodiment, the set of phase differences may be selected from one of 0, π / 2, −π / 2, and π.

[0023] The secondary laser 204 seeded by the primary laser 202 will output a sequence of pairs of pulses having the same phase difference as the sequence of pulses output by the primary laser 202 .

[0024] Pulse injection seeding occurs whenever the secondary laser 204 is switched above its lasing threshold. In this case, the generated secondary light pulse has a fixed phase relationship to the injected primary light pulse. Because only one secondary light pulse is generated for each injected primary light pulse, the phase relationship between the pulses output by the secondary laser 204 is the same as the relationship between the pulses injected into the secondary laser 204.

[0025] Under operating conditions described below in connection with FIG. 3, the secondary laser 204 generates a new sequence of pulses, comprising a sequence of pairs. The phase between pulses in the same pair is randomly selected from one of a set of phase differences, and there is a random phase difference between pulses from different pairs. These pulses will also have a smaller time jitter τ′<τ relative to the pulses output by the primary laser 202. This reduced jitter time therefore improves interference visibility due to the low time jitter of the secondary light pulses.

[0026] For pulsed injection seeding to occur, the frequency of the light pulses from the primary laser 202 must match within a certain range the frequency of the secondary laser 204. In one embodiment, the difference between the frequency of the light provided by the primary laser 202 and the frequency of the secondary laser 204 is less than 30 GHz. In some embodiments, when the secondary laser 204 is a distributed feedback (DFB) laser diode, the frequency difference is less than 100 GHz.

[0027] For pulsed injection seeding to be successful, the relative power of the output optical pulses of primary laser 202 entering the optical cavity of secondary laser 204 must be within certain limits that depend on the type of light source used. In one embodiment, the optical power of the injected optical pulses is at least 1000 times less than the optical output power of secondary laser 204. In one embodiment, the optical power of the injected optical pulses is at least 100 times less than the optical output power of secondary laser 204.

[0028] In one embodiment, the secondary laser 204 and the primary laser 202 are electrically driven gain-switched semiconductor laser diodes. In one embodiment, the secondary laser 204 and the primary laser 202 have the same bandwidth. In one embodiment, both lasers 202, 204 have a bandwidth of 10 GHz. In one embodiment, both lasers 202, 204 have a bandwidth of 2.5 GHz. Here, bandwidth refers to the highest achievable bit rate using a gain-switched laser diode under direct modulation. Lasers of a certain bandwidth can be operated at a lower clock rate.

[0029] 3A is a schematic diagram of a drive scheme for a phase-randomized light source 300 in which both the primary laser 302 and the secondary laser 304 are driven using a single gain modulation unit 308. The gain modulation unit 308 and delay line 310 are an example of a controller configured to apply a time-varying drive signal to the secondary laser 304 so that only one optical pulse is generated during each time period in which an optical pulse is received. The primary laser 302 is connected to the secondary laser 304 via an optical connection 306. The optical connection 306 may be a waveguide, such as an optical fiber. Alternatively, the optical pulses may travel between the primary laser 302 and the secondary laser 304 through free space. The optical connection may include additional components, such as an optical circulator or beam splitter, as in the arrangement of FIG. 2A.

[0030] The gain modulation unit 308 drives both the primary laser 302 and the secondary laser 304 to generate pulses of light. A delay line 310 is used to synchronize the devices. The delay line can be, for example, a fixed-length cable. The gain modulation unit is directly connected to the primary laser 302. For example, if the primary laser 302 is a semiconductor laser, the gain modulation circuit is electrically connected to the primary laser 302. The gain modulation unit 308 is connected to the secondary laser 304 through the delay line 310.

[0031] FIG. 3B shows the time sequence for the single gain modulation scheme shown in FIG. 3A. The top graph shows the gain modulation applied to the primary laser 302. The current applied to the laser is shown on the vertical axis, and time is shown on the horizontal axis. The gain modulation is a time-varying drive signal in the form of a square wave, which, when applied to the primary laser 302, increases the carrier density above and below the lasing threshold. In other words, the gain modulation is a series of pulses. Between the pulses, the gain has a minimum value, which is the gain bias, shown by the dotted line. The wave in this case is a square-type waveform. Different gain modulation signals can be used, such as a sinusoidal wave or a non-periodic time-varying signal. In this case, the current is not reduced to zero between the current modulation pulses, but only to the bias value (shown by the dotted line).

[0032] A current modulation signal is applied to the laser, periodically switching the laser's gain above and below the lasing threshold. The second graph shows the carrier density of the laser on the vertical axis against time on the horizontal axis. The lasing threshold is indicated by the horizontal dashed line. When a current modulation pulse is applied to the laser, the injected carriers increase the carrier density, resulting in an increase in photon density.

[0033] The laser output generated by the modulation signal is shown in the graph below. The vertical axis represents laser intensity, and the horizontal axis represents time. The laser outputs light when the carrier density is above the lasing threshold. Photons generated by spontaneous emission inside the laser cavity are sufficiently amplified by stimulated emission to produce the output signal. The length of the delay between the application of the current modulation pulse and the generation of the output light depends on several parameters, such as the laser type, cavity length, and pumping power.

[0034] The rapid increase in photon density causes a decrease in carrier density. This, in turn, decreases photon density, which increases carrier density. At this point, the current modulation pulse is timed to switch back down to the DC bias level, and the laser emission quickly extinguishes. The laser output therefore consists of a train of short laser pulses, as shown in the graph below.

[0035] To generate longer pulses, the gain bias is chosen closer to the lasing threshold. This means that the carrier density exceeds the lasing threshold sooner, giving the light pulse more time to evolve. Initially, the light intensity overshoots, rapidly reducing the carrier density. This in turn reduces the photon density and increases the carrier density, which in turn increases the light intensity. This competing process causes oscillations in the light intensity at the beginning of the pulse, which are strongly damped and quickly reach a steady state where the intensity is constant. This oscillation is called relaxation oscillation. When the current pulse ends, the laser pulse ends and the current is switched back to the bias value.

[0036] The following graph shows the output of the primary laser 302. Each time the carrier density increases above the lasing threshold, one optical pulse is output. As explained above, there may be a delay between when the gain increases and when the optical pulse is output. The optical pulses output from the primary laser have a large time jitter τ.

[0037] The next graph shows the gain modulation applied to the secondary laser 304. The gain modulation is the same as that applied to the primary laser 302, but with a time delay, labeled with an arrow. The gain modulation is a time-varying drive signal applied to the secondary laser. In other words, the gain modulation applied to the secondary laser 304 is shifted in time with respect to the gain modulation applied to the primary laser 302. Each periodic increase in gain is applied to the secondary laser 304 later than it is applied to the primary laser 302. The delay in this case is approximately half the period of the gain modulation signal. The delay means that the periodic increase in gain is applied to the secondary laser 304 after the optical pulse is injected. Therefore, the optical pulse from the primary laser 302 is present in the laser cavity of the secondary laser 304 when the gain increase is applied, causing the secondary laser 304 to generate an optical pulse by stimulated emission from the primary optical pulse. This means that the generated optical pulse from the secondary laser 304 has a fixed phase relationship with the optical pulse injected from the primary laser 302 into the secondary laser 304.

[0038] After the optical pulse from the primary laser 302 is injected, the secondary laser 304 is switched above the lasing threshold, which initiates a pulse from the secondary laser 304 by stimulated emission caused by the injected optical pulse. The timing of the onset of the gain bias of the secondary laser 304 is controlled via a delay line 310. The last graph shows the output of the secondary laser 304. Only one optical pulse is output each time the carrier density increases above the lasing threshold. As before, there may be a delay between the increase in gain modulation and the output optical pulse. The time jitter of the output optical pulse from the secondary laser 304 is lower than that of the optical pulse from the primary laser 302.

[0039] 3A, gain modulation unit 308 applies time-varying gain modulation to secondary laser 304 so that it switches above lasing threshold only once during the time that each optical pulse from primary laser 302 is incident. The switching of secondary laser 304 is synchronized with the arrival of the optical pulse from primary laser 302 because the same gain modulation signal is applied to both sources, and delay line 310 delays the application of the gain increase to secondary laser 304 relative to primary laser 302.

[0040] In the system shown in Figure 3B, the time-varying gain modulated signal has a rectangular type waveform, however, the time-varying gain modulated signal may comprise a signal having any pulse shape.

[0041] When the light source is a gain-switched semiconductor laser, the gain modulation signal is an applied current or voltage. In one embodiment, the gain modulation signal is an applied current or voltage having a rectangular-type waveform. In an alternative embodiment, the time-varying current or voltage is an electrical sine wave generated by a frequency synthesizer. In one embodiment, the frequency of the gain modulation signal is less than or equal to 4 GHz. In one embodiment, the frequency is 2.5 GHz. In one embodiment, the frequency is 2 GHz.

[0042] A gain-switched semiconductor laser has a good extinction ratio between the "off" state and the state when a pulse is emitted. It can be used to generate ultrashort pulses. In one embodiment, the duration of each pulse output from the secondary laser 304 is less than 200 ps. In one embodiment, the duration of each pulse output from the secondary laser 304 is less than 50 ps. In one embodiment, the duration of each pulse output from the secondary laser 304 is on the order of a few picoseconds. In one embodiment, if the time-varying current or voltage is a square wave current or voltage with a frequency of 2 GHz, the short optical pulses are spaced 500 ps apart.

[0043] In the light source 300 shown in these figures, the primary laser 302 and the secondary laser 304 share the same electrical driver for gain modulation. However, the primary laser 302 and the secondary laser 304 can also be driven by a separate gain modulation unit 308. By driving the gain modulation by a separate unit, it is possible to generate longer optical pulses output from the primary laser 302 than those shown in FIG. 3B because the gain bias value is closer to the lasing threshold. This means that the carrier density exceeds the lasing threshold sooner, giving the optical pulse more time to evolve. This can also be used to reduce jitter.

[0044] FIG. 4 illustrates a laser injection locking device according to one embodiment.

[0045] As will be explained in more detail below with respect to FIG. 4, this configuration uses only a phase retarder to mitigate back reflections, without the need for a bulkier and more expensive Faraday isolator.

[0046] Additionally, it should be noted that the Faraday effect is strongly wavelength-dependent via the Verdet constant, causing Faraday isolators to provide adequate isolation only within a fairly narrow wavelength range. In fact, over some wavelength ranges (e.g., the mid-infrared), materials with suitable Verdet constants are not available, and as a result, Faraday isolators cannot be fabricated at all. In contrast, widely available phase retarders such as quarter-wave plates (especially achromatic quarter-wave plates) and Fresnel rhombohedrons can operate over significantly wider spectral ranges.

[0047] Furthermore, Faraday isolators require a strong B-field to achieve the Faraday effect (e.g., many commercial Faraday isolators include a strong permanent magnet). This can be inconvenient for anyone working nearby with metal parts and / or tools, and in some applications may interfere with other equipment in use nearby. Additionally, magnets generally cannot be miniaturized, resulting in optical isolators that tend to be relatively large and bulky. Magnets also cannot be included in photonics integration schemes, and therefore chip-based technologies that require magnets are generally not possible. In contrast, typical phase retarders do not use a B-field.

[0048] In one embodiment, the secondary laser comprises an input operable to reflect a portion of light of the second polarization that reaches the secondary laser, and the phase retarder is positioned such that light reflected back along the optical path by the input passes through the phase retarder again and is converted into light of a third polarization that is orthogonal to the first polarization.

[0049] In this embodiment, the back-reflected light that re-enters the primary laser is not of the correct polarization to interfere with the operation of the primary laser. For example, if the first polarization is, for example, horizontal, the back-reflected light that re-enters the primary laser will be vertical. If the primary laser has a fixed polarization, such as a semiconductor laser, this vertically polarized light will not be significantly amplified by the optical cavity of the primary laser.

[0050] In one embodiment, the first polarization is linearly polarized and the phase retarder comprises a quarter wave plate, the axis of the quarter wave plate being aligned at 45° to the first polarization.

[0051] Quarter-wave plates are widely available components that are cheaper and smaller than Faraday isolators. Aligning the quarter-wave plate at 45° to the first polarization ensures the conversion of linearly polarized light to circularly polarized light.

[0052] In one embodiment, the quarter wave plate is an achromatic quarter wave plate.

[0053] This allows for effective operation over a wider range of wavelengths.

[0054] In one embodiment, the phase retarder comprises a Fresnel rhombohedron.

[0055] The Fresnel rhomb achieves a polarization shift that is particularly wavelength insensitive, and therefore this embodiment can increase the range of wavelengths over which the device can effectively operate.

[0056] In one embodiment, the apparatus further comprises a polarizer positioned between the primary laser and the phase retarder, the polarizer configured to transmit light of the first polarization.

[0057] This allows for deflection of the back-reflected light of a third polarization so that it does not reach the primary laser.

[0058] In one embodiment, the primary and secondary lasers are configured to operate as pulsed lasers.

[0059] In one embodiment, at least one of the primary laser and the secondary laser is configured to operate using gain switching.

[0060] This allows very high pulse rates (up to GHz) to be used with each phase randomized pulse. Gain-switched laser systems are often extremely reliant on preventing back reflections, and therefore the present invention is particularly advantageous.

[0061] In one embodiment, at least one of the primary laser and the secondary laser is a tunable laser.

[0062] This allows for broadband operation, potentially over a wavelength range of 100 nm or more. In a further embodiment, this is used when the phase retarder is an achromatic quarter wave plate or a Fresnel rhombohedron.

[0063] In one embodiment, the polarization extinction ratio of the light emitted by the primary laser is at least 20 dB, in favor of the first polarization.

[0064] Thus, if the primary laser operates predominantly with a single linear polarization, back reflection of the orthogonal linear polarization will not interfere with the operation of the primary laser.

[0065] In one embodiment, the apparatus further comprises a secondary phase retarder in the optical path after the secondary laser, wherein the secondary phase retarder is configured to convert light of the first polarization to light of the second polarization.

[0066] This mitigates back reflections from any components that may be positioned after the secondary laser, thereby preventing destabilization of the secondary laser.

[0067] In one embodiment, the apparatus further comprises a polarizer in an optical path between the secondary laser and the secondary phase retarder, wherein the polarizer is configured to transmit light of the first polarization.

[0068] This prevents back reflections that pass through the secondary phase retarder from reaching the secondary laser because they will be of orthogonal polarization to the first polarization and will consequently be filtered out by the second polarizer.

[0069] In one embodiment, the optical path is a fiber pathway.

[0070] All-fiber assemblies are generally more compact and robust than assemblies that use free-space transmission.

[0071] In one embodiment, the first-order phase retarder comprises a strained section of optical fiber.

[0072] This can enable quarter wave plate type behavior in an all-fiber assembly.

[0073] In a further embodiment, a packaged micro-optics assembly is provided that includes the above-described device.

[0074] This embodiment provides an extremely compact optical injection locking device while mitigating back reflections. Note that such micro-optical assemblies cannot include Faraday isolators because they would be too large, and therefore the ability to provide a micro-optical assembly is unique to the present invention.

[0075] In a further embodiment, there is provided a quantum cryptography system comprising an emitter and a receiver, the emitter comprising the apparatus described above.

[0076] This embodiment provides a quantum cryptography system that is highly stable and reliable, without requiring the use of a Faraday isolator, as back reflections do not destabilize the emitter's primary laser, which in turn allows quantum cryptography systems such as those described herein to be more compact and to operate over a wider range of wavelengths.

[0077] In a further embodiment, a method of laser injection locking is provided, the method comprising receiving light from a primary laser, the light having a first polarization, converting the light of the first polarization to light of a second polarization, and transmitting the light of the second polarization onto and injecting it into a secondary laser, wherein the first polarization is one of linear polarization and circular polarization and the second polarization is the other.

[0078] This allows for mitigation of back reflections (as described below with respect to FIG. 2) without the need for a Faraday isolator, but instead using a simple shift in polarization that can be achieved with simpler components.

[0079] In one embodiment, converting light of the first polarization to light of the second polarization is performed using a phase retarder.

[0080] This has the advantage that phase retarders are generally smaller, cheaper, less wavelength dependent and available at more operating wavelengths than Faraday isolators.

[0081] In one embodiment, the method further comprises receiving back-reflected light from an input of the secondary laser, converting the back-reflected light to light of a third polarization, the third polarization being orthogonal to the first polarization, and transmitting the light of the third polarization onto the primary laser.

[0082] This means that the back-reflected light that re-enters the primary laser will not be of the correct polarization to interfere with the operation of the primary laser. For example, if the first polarization is, say, horizontal, the back-reflected light that re-enters the primary laser will be vertical. If the primary laser has a fixed polarization, such as a semiconductor laser, this vertically polarized light will not be significantly amplified by the primary laser's optical cavity.

[0083] In one embodiment, converting the back-reflected light to light of the third polarization is done using a phase retarder.

[0084] This embodiment provides a compact and simple optical injection locking device while still achieving the goal of mitigating back reflections.

[0085] 4 illustrates an optical injection synchronization apparatus 400 according to one embodiment of the present disclosure. The apparatus 400 includes a primary laser 402 and a secondary laser 404, with an optical path between the two lasers 402, 404. A phase retarder 410 is provided in the optical path. Although other components are not shown in FIG. 4, the phase retarder 410 is not necessarily the only optical component between the two lasers; in embodiments, other components may also be present.

[0086] 4 is in operation, primary laser 402 emits light 406 that is used to inject secondary laser 404. The secondary laser then emits its own light 408. A phase retarder 410 is provided between primary laser 402 and secondary laser 404, such that light 406 passes through phase retarder 410.

[0087] We now explain how the device in FIG. 4 mitigates the effects of back reflections.

[0088] For simplicity of explanation, it will be assumed that the primary laser 402 emits linearly polarized light 406. It will be understood that the following discussion applies mutatis mutandis if the primary laser 402 instead emits circularly polarized light 406. It should be further noted that the reference to the primary laser 402 emitting linearly polarized light does not necessarily imply that this light is of perfectly uniform polarization; in a practical system, components of one or more other polarizations will also be present. One polarization may significantly dominate; for example, the "dominant" polarization may be dominant, and a polarization extinction ratio of 20 dB or more may be present.

[0089] Phase retarder 410 is configured to convert linearly polarized light to circularly polarized light of light 406. In embodiments in which phase retarder 410 is a quarter-wave plate, this may be achieved by positioning the axis of the quarter-wave plate at 45° to the plane of polarization of light 406, such that a phase shift of π / 2 is introduced between components of the electric field of light 406 parallel to each axis.

[0090] Circularly polarized light can be viewed as a mixture of two orthogonal linearly polarized light beams with a relative phase of π / 2. If the secondary laser 404 is a laser—e.g., a semiconductor laser—that lases with a fixed linear polarization, only one of the two linear components of the circularly polarized light 406 will be significantly amplified by the secondary laser 404, while the other linear component has essentially no effect on the secondary laser 404. Therefore, the secondary laser 404 can be injection-locked with circularly polarized light just as effectively as with linearly polarized light.

[0091] In an embodiment, a portion of the circularly polarized light 406 may be reflected back at the input (e.g., entrance facet) of the secondary laser 404. It is well known that reflecting a wave produces a phase shift of π and a concomitant change in the handedness of the circularly polarized light. For example, if the light 406 is left-handed circularly polarized after the phase retarder 410, it will become right-handed circularly polarized after being reflected back.

[0092] As a result of this phase shift upon reflection, when the back-reflected light reaches the phase retarder 410 it will be converted to linearly polarized light with a linear polarization orthogonal to that emitted by the primary laser 402. For example, if the primary laser 402 emits horizontal light, any back-reflected light returning to the primary laser (having passed through the phase retarder 410 twice and reflected) will be vertically polarized.

[0093] As a result, if primary laser 402 is a laser (such as a semiconductor laser) that lases with a fixed linear polarization, these back reflections that re-enter primary laser 420 will be of the wrong polarization to be amplified and will not significantly affect the operation of primary laser 402.

[0094] In this way, the phase retarder 410 mitigates the effects of back reflections, allowing the primary laser 402 to operate, for example, without loss of phase coherence.

[0095] Note that, unlike the Faraday isolator described above with respect to FIG. 1 , this method does not function regardless of the polarization of the light 406 (or the back-reflected light). Rather, the light 406 emitted by the primary laser 402 must have a known dominant polarization, the phase retarder 410 must be aligned to account for this polarization, and additionally, the polarization of the back-reflected light must have a predictable relationship to the polarization of the light 406. Notably, the reason B-field nonreciprocity is not required (as is required for a Faraday isolator) is due to the π phase shift upon back-reflection and the concomitant change in handedness of circularly polarized light. This allows for avoiding potential interference and disadvantages with other components that can result from having a strong B-field.

[0096] As mentioned above, the primary laser 402 may emit light 406 that is circularly polarized rather than linearly polarized. In this case, the phase retarder 410 will convert this circularly polarized light to linearly polarized light. Therefore, the light emitted by the primary laser 402 is either circular or linear, and the light after the phase retarder 410 is said to be the other of these two options.

[0097] The device of Figure 4 may be implemented equally well as an all-fiber assembly with or without free-space transmission of light. All-fiber embodiments may allow for very compact and robust assemblies.

[0098] There are several different components that can function as the phase retarder 410. In a free-space assembly, a quarter-wave plate (optionally an achromatic quarter-wave plate for an increased wavelength range) or a Fresnel rhombohedron can be used. In an all-fiber assembly, a strained section of fiber can function as the phase retarder 410, with the angle of the fiber and the applied strain chosen to achieve quarter-wave plate-like behavior.

[0099] In an all-fiber setup, the portion of the optical path where the polarization is linear may use polarization-maintaining (PM) fiber, while the portion with circular polarization may require alternative techniques. For example, circular polarization may be preserved using a spun optical fiber "pigtail," meaning PM fiber is rotated as a preform during the fiber drawing manufacturing process. This results in a fiber that can preserve circular polarization.

[0100] It should be further noted that the apparatus of Figure 4 operates equally well regardless of whether the primary laser 402 and / or secondary laser 404 are operated as pulsed or CW lasers. In many quantum cryptography systems, it may be more desirable to operate them as pulsed lasers, potentially using gain switching. Gain switching refers to an arrangement in which the primary laser 420 is controlled by a control unit to generate high-speed (e.g., GHz) pulses, with each pulse having a random phase seeded by random vacuum fluctuations. In such embodiments, the secondary laser 404 is often controlled by the same control unit.

[0101] The wider operating range of wavelengths achieved by the apparatus of Figure 4 also makes this assembly highly suitable when the primary laser 402 and / or secondary laser 404 are tunable lasers, meaning that their wavelengths can be tuned over a range. For example, one or both of the primary laser 402 and secondary laser 404 can be external cavity diode lasers, where the cavity includes a tunable element such as an adjustable mirror or filter. Note that such tunable lasers typically vary over a significant range of wavelengths (100 nm or more), which is too large for a Faraday isolator but can be achieved using an achromatic quarter-wave plate and / or a Fresnel rhombohedron as the phase retarder 410, as described above.

[0102] The device of Figure 4 can be included in a packaged micro-optical assembly if a very compact assembly is desired for a particular application. Note that this is made possible, among other things, by the omission of bulky Faraday isolators; phase retarders such as wave plates and Fresnel rhombohedrons can be more easily miniaturized. Such micro-optical assemblies can be fiber-coupled for use in fiber-based miniature optical setups.

[0103] FIG. 5 shows an assembly 500 similar to the assembly 400 of FIG. 4 with the addition of a polarizer 502 between the primary laser 402 and the phase retarder 410 .

[0104] It is envisioned that polarizer 502 is configured to transmit light 406 emitted by primary laser 402. For example, primary laser 402 may emit primarily horizontally polarized light, and polarizer 502 may be angled to transmit horizontally polarized light.

[0105] 4, the back-reflected light is converted by the phase retarder 410 to have a polarization that is orthogonal to the polarization emitted by the primary laser 402. For example, the primary laser 402 emits horizontally polarized light, and the back-reflected light is converted by the phase retarder 410 to have a vertical polarization.

[0106] As a result, the back-reflected light returning to polarizer 502 will be filtered out (e.g., absorbed or deflected) because polarizer 502 is configured to transmit horizontally polarized light (in this illustrative example) and the back-reflected light has vertical polarization.

[0107] Thus, polarizer 502 has the effect of significantly reducing (or effectively eliminating) the back-reflected light that reaches primary laser 402 .

[0108] As mentioned above, if the primary laser 402 has a fixed polarization axis (e.g., if the primary laser 402 is a semiconductor laser), the light from the back reflection will not interfere with the operation of the primary laser due to the orthogonal polarization. However, if the primary laser 402 is not such a laser, or if further assurance is desired, a polarizer 502 can be added. In particular, the polarizer 502 can function to prevent not only interferometric destabilization of the primary laser 402, but also cavity population dynamic based destabilization.

[0109] In a free-space assembly, the polarizer 502 can be, for example, a polarizing beam splitter, or further miniaturization can be achieved by using, for example, a wire grid. If the device is an all-fiber assembly as described above with respect to Figure 4, the polarizer can be implemented using a section of polarizing optical fiber, such as a fiber with embedded stress rods that induce extreme birefringence, resulting in the fiber acting as a waveguide for only one polarization.

[0110] Figure 6 shows an apparatus 600 similar to apparatus 500 of Figure 5 with the addition of a secondary polarizer 604 and a secondary phase retarder 602 after the secondary laser 404. The secondary phase retarder 602 and secondary polarizer 604 may be added to mitigate the effects of back reflections from components after the secondary laser 404 that could otherwise destabilize the secondary laser 404. The operation of the secondary phase retarder 602 and secondary polarizer 604 is exactly like the operation of the phase retarder 410 and polarizer 502 described above for the primary laser 402, but instead protects the secondary laser 404.

[0111] It is assumed that the secondary polarizer 604 will be oriented to transmit light of the dominant polarization of the secondary laser 404, which in most optical injection locking devices is likely to be the same as the dominant polarization of the primary laser 402.

[0112] It will be appreciated that one or both of polarizers 502 and 604 may be omitted from assembly 600, as polarization of back reflections may not necessarily be required depending on the operation of primary laser 402 and secondary laser 404 (as discussed above).

[0113] It is further noted that additional components may be present in any of apparatus 400, 500, or 600. For example, additional components may be present between primary laser 402 and secondary laser 404, such as a pickoff plate for intensity monitoring, or a variable optical attenuator (VOA) for controlling the injection power of secondary laser 404. For example, a VOA may be used to set the injection power to secondary laser 404 to be 10-15 dB lower than the free-running emission power of secondary laser 404.

[0114] FIG. 7 illustrates steps of a method 700 of laser injection locking.

[0115] In step S702, light of a first polarization (linearly or circularly polarized) is received from a primary laser.

[0116] In step S704, the light of the first polarization is converted to light of a second polarization, which is either linearly polarized or circularly polarized. In other words, if the received light has linear polarization, it is converted to circular polarization, while if the received light has circular polarization, it is converted to linear polarization. It will be appreciated that in the illustrative example apparatus of Figures 4-6 as described above, this step is accomplished by the phase retarder 410.

[0117] In step S706, as part of the optical injection locking device, light of the second polarization is transmitted onto and injected into the secondary laser.

[0118] Based on the above description of Figures 4-6, in embodiments in which the secondary laser input back-reflects a portion of light of the second polarization, this back-reflected light can then be converted to light of a third polarization that is orthogonal to the first polarization. For example, if the first polarization is horizontally polarized and the second polarization is circularly polarized, the third polarization can be vertically polarized. This can be achieved, for example, by the same phase retarder that converts the first polarization to the second polarization (as in Figures 4-6 above).

[0119] Next, a basic quantum communication protocol using polarized light will be described. This can be used for transmission between a transmitter and a receiver. However, it should be noted that this is not meant to be limiting, and other protocols can also be used. Furthermore, the above system can be used with any QKD system and is not limited to use with polarized light. For example, phase or energy / time based QKD protocols can also be used.

[0120] The protocol uses two bases, each described by two orthogonal states. In this example, these bases are horizontal / vertical (H / V) and diagonal / anti-diagonal (D / A) polarization. However, a left-handed / right-handed circular polarization (L / R) basis could also be chosen.

[0121] The transmitter in the protocol prepares a state with one of H, V, D, or A polarization. In other words, the prepared state is selected from two orthogonal states (H and V or D and A) in one of two bases, H / V and D / A. This can be thought of as sending signals of 0 and 1 in one of the two bases, for example, H=0, V=1 in the H / V basis, and D=0, A=1 in the D / A basis. The pulse is attenuated to comprise less than one photon on average. Therefore, if a measurement is made on the pulse, it will be destroyed. It is also impossible to split the pulse.

[0122] The receiver uses a measurement basis for the polarization of the pulse, selected from the H / V basis or the D / A basis. The selection of the measurement basis can be active or passive. In passive selection, the basis is selected using a fixed component such as a beam splitter. In "active" basis selection, the receiver uses, for example, a modulator with an electrical control signal to determine which basis to measure in. If the basis used to measure the pulse at the receiver is the same as the basis used to encode the pulse, the receiver's measurement of the pulse will be accurate. However, if the receiver selects the other basis to measure the pulse, there will be a 50% error in the result measured by the receiver.

[0123] To establish the key, the transmitter and receiver compare the bases used for encoding and measurement (decoding). If they match, the result is kept; if they do not match, the result is discarded. The above method is very secure. If an eavesdropper intercepts the pulses and measures them, the eavesdropper must prepare another pulse to send to the receiver. However, the eavesdropper does not know the correct measurement base and therefore only has a 50% chance of measuring the pulse correctly. Any pulse recreated by an eavesdropper will cause a larger error rate for the receiver, which can be used to prove the presence of an eavesdropper. The transmitter and receiver compare small portions of the key to determine the error rate and therefore the presence of an eavesdropper.

[0124] Although the above has been described with respect to polarization, this is by way of example only: other QKD protocols based on phase, or other systems such as energy / time, may be used.

[0125] QKD requires two channels: a "quantum channel" used for communication of pulses containing on average one photon or less, and a classical channel used for the basis ("sieving") discussion. The classical channel can also be used for further communication once a key is established on the quantum channel. Note that the term "channel" is used to refer to a logical channel. The quantum and classical channels can be located within the same physical fiber.

[0126] Next, an example of a receiver and a transmitter will be described with reference to FIGS. 8A and 8B.

[0127] One example of a possible transmitter is shown in FIG. 8A as 800. This transmitter can be any type of quantum transmitter capable of emitting coded photons. In this particular example, polarization coding is described, but any type of coding can be used, such as phase or other coding types, such as energy / time. In the example of FIG. 8A, transmitter 800 includes four emitters 802, 804, 806, and 808, each of which emits horizontally polarized light. For simplicity, each emitter is shown as a single unit. However, in this embodiment, each emitter includes the optical injection locking device of FIG. 4. The output from emitter 802 is fed to polarization combining optics 810. The output from emitter 804 is fed to polarization combining optics 810 via a half-wave plate configured to convert the horizontally polarized light to diagonally polarized light. The output from emitter 806 is fed through a half-wave plate configured to convert horizontally polarized light to vertically polarized light and towards polarization combining optics 810. The output from emitter 808 is fed through a half-wave plate configured to convert horizontally polarized light to oppositely polarized light and towards polarization combining optics 801.

[0128] The polarization combining optics 810 allows different polarizations to be combined into a stream of pulses with randomly varying polarizations. This can be achieved in many different ways. For example, the lasers can be pulsed lasers, and a controller (not shown) can be provided to randomly select a laser from emitters 802, 804, 806, and 808 to randomly output pulses, so that the pulses reach the polarization combining optics 810 one at a time. In other embodiments, the polarization combining optics 810 or additional components can be configured to randomly select output from one emitter or randomly selectively block output from three emitters to allow a pulsed output stream. The pulses can be generated by a pulsed laser, or a CW laser can be used with additional components to chop the output into pulses.

[0129] An attenuator (not shown) is then used to attenuate the power of the pulses so that they contain, on average, less than one photon.

[0130] The transmitter 800 of FIG. 8A includes four emitters 802, 804, 806, and 808, each of which includes a primary laser and a secondary laser in an optically injection-locked configuration. This optical injection-locking is achieved using any of the setups described above with respect to FIGS. 4-6, where a phase retarder is positioned between each primary laser and its respective secondary laser to mitigate back reflections. Alternatively, the transmitter 800 may include one primary laser optically injecting four secondary lasers in the configuration of the setups of FIGS. 4-6, where a phase retarder is positioned between each primary laser and its respective secondary laser to mitigate back reflections. Finally, the four emitters 802, 804, 806, and 808 may be replaced with a single primary laser and an injection-locked secondary laser, where the secondary laser is followed by a variable polarization rotator instead of the waveplate shown in FIG. 8A. The use of a variable polarization rotator tends to reduce the bit rate of the transmitter 800 because each time the rotator is switched, a delay is introduced.

[0131] A simplified form of receiver 812 is shown in FIG. 8B. Receiver 812 comprises a 50-50 beam splitter 814 that will direct incoming pulses along either a first measurement channel 815 or a second measurement channel 816. Because the pulses contain less than one photon on average, 50-50 beam splitter 814 will randomly direct pulses along one of the first or second measurement channels. This has the consequence of selecting the measurement basis to be either the X(D / A) basis or the Z(H / V) basis. Non-polarizing beam splitter 814 functions to enable the random selection of one of the two bases.

[0132] The first measurement channel 815 is for the X basis, which corresponds to the D / A basis. Here, a half-wave plate 818 is provided to rotate the polarization by 45 degrees between the two detection branches, i.e., to provide two measurement bases, X and Z. The output of half-wave plate 818 is then directed towards a polarizing beam splitter 820, which directs pulses with opposite polarities towards an opposite-angle detector 824 and pulses with diagonal polarities towards a diagonal detector 826. Detectors 824 and 826 are single-photon detectors, such as avalanche photodiodes.

[0133] Pulses directed along second measurement channel 816 are measured in the Z-base to determine whether they are horizontal or vertical. Here, pulses directed into second measurement channel 816 are directed towards polarizing beam splitter 822, which directs vertically polarized pulses towards vertical detector 828 and horizontally polarized pulses towards horizontal detector 830. Again, detectors 828 and 830 are single-photon detectors.

[0134] If a photon polarized in the D / A basis is received and randomly sent to be measured in the Z basis along the second measurement channel 816, it is likely that one of the detectors 828, 830 will register a count. However, this result is unreliable because there is a 50 / 50 chance that a photon received at the polarizing beam splitter 822 will be directed towards either the vertical or horizontal detector.

[0135] While certain specific embodiments have been described, these embodiments are presented by way of example only and are not intended to limit the scope of the invention. Indeed, the novel devices and methods described herein may be embodied in a variety of other forms, and various omissions, substitutions, and changes in the forms of the devices, methods, and articles of manufacture described herein may be made without departing from the spirit of the invention. The accompanying claims and their equivalents are intended to cover such forms or modifications as fall within the scope and spirit of the invention.

Claims

1. a primary laser configured to emit light of a first polarization; a secondary laser configured to be injected with light from the primary laser, the secondary laser being connected to the primary laser by an optical path; a primary phase retarder disposed in the optical path between the primary laser and the secondary laser, the primary phase retarder configured to convert the light of the first polarization to light of a second polarization and to convert light of the first polarization reflected back from the input of the secondary laser to light of a third polarization orthogonal to the first polarization. Equipped with the light of the third polarization is transmitted onto the primary laser; The laser injection locking apparatus, wherein the first polarization is one of linear and circular polarization and the second polarization is the other.

2. the secondary laser having an input operable to reflect a portion of the light of the second polarization that reaches the secondary laser; 2. The device of claim 1, wherein the first-order phase retarder is positioned such that light reflected back along the optical path by the input passes through the first-order phase retarder again and is converted to light of the third polarization.

3. 10. The apparatus of claim 1, wherein the first polarization is the linear polarization and the first-order phase retarder comprises a quarter-wave plate, the axis of the quarter-wave plate being aligned at 45° to the first polarization.

4. 10. The apparatus of claim 1, further comprising a polarizer positioned between the primary laser and the primary phase retarder, the polarizer configured to transmit the light of the first polarization.

5. The apparatus of claim 1 , wherein the primary laser and the secondary laser are configured to operate as multiple pulse lasers.

6. 6. The apparatus of claim 5, wherein at least one of the primary laser and the secondary laser is configured to operate using gain switching.

7. The apparatus of claim 1 , wherein at least one of the primary laser and the secondary laser is a tunable laser.

8. 10. The apparatus of claim 1, further comprising a secondary phase retarder in the optical path after the secondary laser, wherein the secondary phase retarder is configured to convert light of the first polarization to light of the second polarization.

9. 10. The apparatus of claim 8, further comprising a polarizer in the optical path between the secondary laser and the secondary phase retarder, wherein the polarizer is configured to transmit the light of the first polarization.

10. The apparatus of claim 1 , wherein the optical path is a fiber path.

11. The apparatus of claim 10 , wherein the first-order phase retarder comprises a strained section of optical fiber.

12. A packaged micro-optical assembly comprising the device of claim 1.

13. A quantum cryptography system comprising an emitter and a receiver, wherein the emitter comprises the apparatus of claim 1.

14. receiving light from a primary laser, the light having a first polarization; converting the light of the first polarization to light of a second polarization using a phase retarder; transmitting the light of the second polarization onto a secondary laser and injecting the light into the secondary laser; receiving back-reflected light from the input of the secondary laser; converting the back-reflected light into light of a third polarization, the third polarization being orthogonal to the first polarization; transmitting the light of the third polarization onto the primary laser; Equipped with The method of injection locking a laser, wherein the first polarization is one of linear polarization and circular polarization, and the second polarization is the other.

15. The method of claim 14 , wherein the converting of the back-reflected light to the light of the third polarization is performed using the phase retarder.

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