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IEEE Transactions on Circuits & Systems II Express Briefs

Laser Diode Current Driver With $(1-t/T)^{-1}$ Time Dependence in 0.35- $\mu\text{m}$ BiCMOS Technology for Quantum Random Number Generators

Nikša Tadić, Bernhard Goll, Horst Zimmermann

Published Jun 29, 2016• 4 citations
Publisher

Abstract

A laser diode current driver with (1-t/T)-1time dependence in 0.35-μm BiCMOS technology is presented in this brief. It is intended for application in optical quantum random number generators. The current driver is based on the balanced type of a bipolar junction transistor translinear loop with a sawtooth current as one of its inputs. The controllability of the frequency, shape, amplitude, and dc level of the current pulses is provided. The achievable frequency bandwidth of the current pulses is up to 100 MHz. A relative error of the measured waveform at a frequency of 25 MHz is smaller than 3% in 97.4% of the time period of interest.

DOI
10.1109/tcsii.2016.2586381

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Laser Diode Current Driver With (1−t/T)−1(1-t/T)^{-1}(1−t/T)−1 Time Dependence in 0.35- μm\mu\text{m}μm BiCMOS Technology for Quantum Random Number Generators
A laser diode current driver with (1-t/T)-1time dependence in 0.35-μm BiCMOS technology is presented in this brief. It is intended for application in optical quantum random number generators. The current driver is based on the balanced type of a bipolar junction transistor translinear loop with a sawtooth current as one of its inputs. The controllability of the frequency, shape, amplitude, and dc level of the current pulses is provided. The achievable frequency bandwidth of the current pulses is up to 100 MHz. A relative error of the measured waveform at a frequency of 25 MHz is smaller than 3% in 97.4% of the time period of interest.
Quantum key distribution (QKD) has rapidly developed recently. The repetition frequency of the QKD system increases from tens or hundreds of MHz to GHz. A laser diode (LD) operating in gain-switched mode is widely used as a weak coherent light source in a QKD system. We present an LD driver circuit fabricated in 0.18 μm SiGe heterojunction bipolar transistor bipolar complementary metal oxide semiconductor technology. The circuit can operate at frequencies up to 2.5 GHz, satisfying the requirements of high-speed laser drives in practical QKD systems. The output current of the driver circuit can reach 100 mA when driving an LD whose input equivalent resistance is ∼21 Ω. The extinction ratio of the 1550-nm distributed feedback LD light source driven by our driver circuit reaches 23 dB at the operating frequency of 2.5 GHz, meeting the requirements of QKD systems. This circuit will be used in miniaturized QKD systems.
Experimental demonstration of a quantum random number generator based on one single-photon avalanche diode (SPAD) detector, a T / ( T − t ) pulse-shaped laser, and an field-programmable gate array (FPGA) acquisition module is presented. An integrated laser driver drives an external laser diode at 670 nm wavelength, whereas the SPAD with a photon detection probability of 18.5% is integrated together with an active quenching-resetting circuit. The SPAD detector generates counts for the interarrival time (IAT) measurement system implemented in an FPGA, where the change of the IATs between consecutive pulses is used to derive a random bit stream. It is shown how the application of a pulse-shaped laser driver can increase the performance of the system as compared to the continuous-wave operation of the laser diode to achieve the maximum generation rate of 5 Mbps while using a single SPAD. The generated numbers pass all randomness tests of the National Institute of Standards and Technology (NIST), Dieharder, and ENT test (pseudorandom number sequence test) suits.
A series of simple and low-cost devices for switching, amplifying, and chirping diode lasers based on current modulation are presented. Direct modulation of diode laser currents is rarely sufficient to establish precise amplitude and phase control over light, as its effects on these parameters are not independent. These devices overcome this limitation by exploiting amplifier saturation and dramatically outperform commonly used external modulators in key figures of merit for quantum technological applications. Semiconductor optical amplifiers operated on either rubidium D line are recast as intensity switches and shown to achieve ON:OFF ratios >106 in as little as 50 ns. Current is switched to a 795 nm wavelength (Rb D1) tapered amplifier to produce optical pulses of few nanosecond duration and peak powers of 3 W at a similar extinction ratio. Fast rf pulses are applied directly to a laser diode to shift its emission frequency by up to 300 MHz in either direction and at a maximum chirp rate of 150 MHz ns-1. Finally, the latter components are combined, yielding a system that produces watt-level optical pulses with arbitrary frequency chirps in the given range and <2% residual intensity variation, all within 65 ns upon asynchronous demand. Such systems have broad application in atomic, molecular, and optical physics and are of particular interest to fast experiments simultaneously requiring high power and low noise, for example, quantum memory experiments with atomic vapors.
Quantum key distribution (QKD) research has yielded highly fruitful results and is currently undergoing an industrialization transformation. In QKD systems, electro-optic modulators are typically employed to prepare the required quantum states. While various QKD systems operating at GHz repetition frequency have demonstrated exceptional performance, they predominantly rely on instruments or printed circuit boards to fulfill the driving circuit function of the electro-optic modulator. Consequently, these systems tend to be complex with low integration levels. To address this challenge, we have introduced a modulator driver integrated circuit in 0.18 µm SiGe BiCMOS technology. The circuit can generate multiple-level driving signals with a clock frequency of 1.25 GHz and a rising edge of ∼50 ps. Each voltage amplitude can be independently adjusted, ensuring the precise preparation of quantum states. The measured signal-to-noise ratio was more than 17 dB, resulting in a low quantum bit error rate of 0.24% in our polarization-encoding system. This work will contribute to the advancement of QKD system integration and promote the industrialization process in this field.
In this study, a compact and low-power-consumption quantum random number generator (QRNG) based on a laser diode and a hybrid chip with integrated silicon photonics is proposed and verified experimentally. The hybrid chip’s size is 8.8 × 2.6 × 1 mm3, and the power of the entropy source is 80 mW. A common-mode rejection ratio greater than 40 dB was achieved using an optimized 1 × 2 multimode interferometer structure. A method for optimizing the quantum-to-classical noise ratio is presented. A quantum-to-classical noise ratio of approximately 9 dB was achieved when the photoelectron current is 1 μA using a balance homodyne detector with a high dark current GeSi photodiode. The proposed QRNG has the potential for use in scenarios of moderate MHz random number generation speed, with low power, small volume, and low cost prioritized.