Avalanche Photodiode Detector Research Articles

A novel integrated time-resolved array avalanche photodiode detection system for nuclear resonant scattering measurements.

The nuclear resonant scattering (NRS) experiment requires photon-counting detectors with high time resolution, short dead time, large dynamic range, low noise, and large detection area. An 8-channel avalanche photodiode (APD) array detector system with high integrity, flexibility, and reliability has been developed to adapt to the demands of NRS experiments. The detector system mainly consists of four key parts: (i) an array-APD sensor, (ii) 8-channel integrated fast preamplifiers, (iii) the time-to-digital converter readout electronics, and (iv) a data acquisition system and EPICS support software. Remarkably, the system exhibits a time resolution of better than 500ps and has a sufficiently low noise level, allowing for the lowest detection energy threshold of 4keV. The performance of the new array-APD system as well as its real application in nuclear forward scattering (NFS) and nuclear resonant inelastic x-ray scattering (NRIXS) experiments was tested in two synchrotron facilities. With the new system, the NFS signal very close to the prompt electronic scattering signal can be extracted. Thanks to the customized EPICS-areaDetector-based control software, NRIXS spectra can be readily measured with time and energy information of the NRIXS signal stored in the raw data, which is promising for developing NRIXS data analysis in the time domain. The array-APD detector can be deployed for nuclear resonant scattering experiments at various synchrotron radiation facilities.

Integrated Encapsulation and Implementation of a Linear-Mode APD Detector for Single-Pixel Imaging Lidar

Single-pixel imaging lidar is a novel technology that leverages single-pixel detectors without spatial resolution and spatial light modulators to capture images by reconstruction. This technique has potential imaging capability in non-visible wavelengths compared with surface array detectors. An avalanche photodiode (APD) is a device in which the internal photoelectric effect and the avalanche multiplication effect are exploited to detect and amplify optical signals. An encapsulated APD detector, with an APD device as the core, is the preferred photodetector for lidar due to its high quantum efficiency in the near-infrared waveband. However, research into APD detectors in China is still in the exploratory period, when most of the work focuses on theoretical analysis and experimental verification. This is a far cry from foreign research levels in key technologies, and the required near-infrared APD detectors with high sensitivity and low noise have to be imported at a high price. In this present study, an encapsulated APD detector was designed in a linear mode by integrating a bare APD tube, a bias power circuit, a temperature control circuit and a signal processing circuit, and the corresponding theoretical analysis, circuit design, circuit simulation and experimental tests were carried out. Then, the APD detector was applied in the single-pixel imaging lidar system. The study showed that the bias power circuit could provide the APD with an operating voltage of DC 1.6 V to 300 V and a ripple voltage of less than 4.2 mV. Not only that, the temperature control circuit quickly changed the operating state of the Thermo Electric Cooler (TEC) to stabilize the ambient temperature of the APD and maintain it at 25 ± 0.3 °C within 5 h. The signal processing circuit was designed with a multi-stage amplification cascade structure, effectively raising the gain of signal amplification. By comparison, the trial also suggested that the encapsulated APD detector and the commercial Licel detector had a good agreement on the scattered signal, such as a repetition rate and pulse width response under the same lidar environment. Therefore, target objects in real atmospheric environments could be imaged by applying the encapsulated APD detector to the near-infrared single-pixel imaging lidar system.

Near-Infrared Emitting of Zero-Dimensional Europium(II) Halide Scintillators: Energy Transfer Engineering via Sm2+ Doping

Near-infrared (NIR)-emitting scintillators, coupled with high quantum efficiency silicon-based photodetectors, have emerged as a promising solution for highly efficient radiation detection applications. However, the study of efficient NIR-emitting scintillators and their associated luminescence mechanism is still limited. In this work, we developed NIR-emitting zero-dimensional Cs4EuBr6 halide scintillators via Sm2+ doping. Single crystals of Cs4EuBr6 with varying Sm2+ concentrations were grown by the vertical Bridgman method. Under X-ray irradiation, the scintillation emission of highly Sm-doped Cs4EuBr6 single crystals is dominated by the Sm2+ 5d–4f emission peaking at 831 nm with a weak Eu2+ 5d–4f emission peaking at 443 nm. The energy transfer processes between Eu2+ and Sm2+ were investigated by using photoluminescence (PL) spectra, PL decay kinetics, and soft X-ray excited decay kinetics measurements. Based on the temperature-dependent PL decay results, we constructed a configurational coordinate diagram of Sm2+ in the Cs4EuBr6 host. Furthermore, we evaluated the gamma spectroscopy response of Cs4EuBr6:Sm single crystals using an avalanche photodiode detector known for its high sensitivity in the NIR wavelength region.

Effect of sub-grain boundaries in CdZnTe on HgCdTe film grown by molecular beam epitaxy

HgCdTe films grown by molecular beam epitaxy (MBE) are essential for creating high-performance infrared focal plane arrays (IRFPAs) like dual band detectors, high operating temperature (HOT) detectors, and avalanche photodiode (APD) detectors. CdZnTe is recognized as the optimal substrates for growing high crystal quality HgCdTe due to its lattice matching, which is adjusted by selecting the Zn mole fraction. However, defects like grain boundaries, twins, inclusions, dislocations, and sub-grain boundaries that often occur in CdZnTe substrates can significantly reduce the crystal quality of HgCdTe. HgCdTe grown on CdZnTe with these defects usually no longer suit for high quality device fabrication. Although the impact from various defects have been extensively studied in HgCdTe grown on CdZnTe, but the sub-grain boundaries is still not yet involved. In our study, to better understand the effect of sub-grain boundaries in CdZnTe on HgCdTe films, CdZnTe substrates with and without sub-grain boundary were selected to growth MWIR HgCdTe films under optimized condition. Results indicated that etch pit density (EPD) of CdZnTe substrates with sub-grain boundary is approximately 3–5 times higher than boundary free areas. But HgCdTe grown on these substrates bears more than an order of magnitude increase in dislocation proliferation compared to the boundary free region. Dislocation generation resulting from dislocation penetration from substrates, mismatch between HgCdTe and CdZnTe, and sub-grain boundaries were investigated and compared. The intense dislocation proliferation in HgCdTe may be mainly due to twins formed during the initial growth stage of HgCdTe films, rather than dislocation penetration from substrates, as revealed by reflection high-energy electron diffraction (RHEED) pattern monitoring. Therefore, screening for sub-grain boundaries in CdZnTe before HgCdTe growth is necessary.

Mid-wave infrared optical receiver based on an InAsSb-nBn photodetector using the barrier doping engineering technique for low-power satellite optical wireless communication.

This paper proposes a new nBn photodetector (nBn-PD) based on InAsSb with a barrier doping engineering technique [core-shell doped barrier (CSD-B) nBn-PD] for utilization as a low-power receiver in satellite optical wireless communication (Sat-OWC) systems. In the proposed structure, the absorber layer is selected from an I n A s 1-x S b x (x=0.17) ternary compound semiconductor. The difference between this structure and other nBn structures is the placement of the top and bottom contacts in the form of a PN junction, which increases the efficiency of the proposed device through the creation of a built-in electric field. Also, a barrier layer is placed from the AlSb binary compound. The presence of the CSD-B layer with the high conduction band offset and very low valence band offset improves the performance of the proposed device compared to conventional PN and avalanche photodiode detectors. By applying -0.1V bias at 125K, the dark current is demonstrated at 4.31×10-5 A/c m 2 by assuming high-level traps and defect conditions. Examining the figure of merit parameters under back-side illumination with a 50% cutoff wavelength of 4.6µm shows that at 150K, the responsivity of the CSD-B nBn-PD device reaches about 1.8A/W under 0.05W/c m 2 light intensity. Regarding the great importance of using low-noise receivers in Sat-OWC systems, the results indicate that the noise, noise equivalent power, and noise equivalent irradiance are calculated as 9.98×10-15 A H z -1/2, 9.21×10-15 W H z 1/2, and 1.02×10-9 W/c m 2, respectively, at -0.5V bias voltage and 4µm laser illumination with the influence of shot-thermal noise. Also, D ∗ obtains 3.26×1011 c m H z 1/2/W without using the anti-reflection coating layer. In addition, since the bit error rate (BER) plays an essential role in the Sat-OWC systems, the effect of different modulations on the BER sensitivity of the proposed receiver is investigated. The results represent that the pulse position modulation and return zero on-off keying modulations create the lowest BER. Attenuation is also investigated as a factor that significantly affects BER sensitivity. The results clearly express that the proposed detector provides the knowledge to achieve a high-quality Sat-OWC system.

Infrared detector module for airborne hyperspectral LiDAR: design and demonstration.

Realizing the integrated acquisition and identification of the elevation information and spectral information of the observation target is at the frontier and a future trend of Earth observation technology. This study designs and develops a set of airborne hyperspectral imaging lidar optical receiving systems and investigates the detection of the infrared band echo signal of the lidar system. A set of avalanche photodiode (APD) detectors is independently designed to detect the weak echo signal of 800-900nm band. The actual radius of the photosensitive surface of the APD detector is 0.25mm. We design and demonstrate the optical focusing system of the APD detector in the laboratory and obtain that the image plane size of the optical fiber end faces of the APD detector from channel 47 to channel 56 is close to 0.3mm. Results show that the optical focusing system of the self-designed APD detector is reliable. On the basis of the focal plane splitting technology of the fiber array, we couple the echo signal of 800-900nm band to the corresponding APD detector through the fiber array and conduct a series of test experiments for the APD detector. Field test results of the ground-based platform show that the APD detectors in all channels can complete the remote sensing measurement of 500m. The development of this APD detector solves the problem of hyperspectral imaging under weak light signals and realizes the accurate detection of ground targets in the infrared band by airborne hyperspectral imaging lidar.

SECALROC2: A Low-Noise, High-Speed, and Full Digital Outputs Front-End ASIC for STCF Electromagnetic Calorimeter With APD Detectors

In this work, a low-noise and high-rate front-end readout application-specific integrated circuit (ASIC) named SECALROC2 for the electromagnetic calorimeter (ECAL) in the super tau-charm factory (STCF) is presented. The chip is designed to process signals generated from avalanche photodiode detectors (APDs) for both energy and time measurement. The large detector capacitance and leakage current of the APD pose a great challenge to the noise and speed performance of the front-end readout circuit (ROC). A low-noise preamplifier and a fifth-order high-linearity shaper with adjustable peaking times are proposed to decrease noise induced by the 270-pF detector capacitance. A peak detect-and-hold circuit and a high-linearity differential analog buffer are adopted to convert each peak voltage of the shaper to differential voltages with sufficient driving capability. A fast shaper and a discriminator are implemented to produce trigger signals for the time of arrival (TOA) measurement. To reduce the complexity and power consumption of the ECAL system, a 12-bit 10-MS/s pipelined analog-to-digital converter (ADC) and a 16-bit three-stage time-to-digital converter (TDC) with 42-ps bin size TDC are integrated on-chip for full-digitalized outputs. This ASIC was designed and fabricated in a standard commercial 1P6M 0.18- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> mixed-signal CMOS process and the chip area occupancy is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.4\times1.6$ </tex-math></inline-formula> mm2. The experimental results show that the input charge of the ASIC ranges from 3.2 to 400 fC. The measured nonlinear error of the system is less than 2.5% with an event rate up to 400 kcps. The equivalent noise charge (ENC) is 1367 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{e}^{-}$ </tex-math></inline-formula> + 4.93 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{e}^{-}$ </tex-math></inline-formula> /pF at 800-ns peaking time. The timing resolution is better than 540 ps at an input charge of 160 fC. The measured ENC can achieve 3820 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{e}^{-}$ </tex-math></inline-formula> when the ASIC is connected to an APD detector. The effective number of bits (ENOB) of the ADC is 10.8-bit when operated at 10 MS/s.

Avalanche photodiode detection system enables X-ray communication up to 10 Mbps

As the core part of the X-ray communication (XCOM) system, the X-ray detector directly affects the data rate and bit error ratio (BER) performance. An avalanche photodiode (APD) with fast response time and high intern gain is chosen as the detector sensor to improve the data rate of XCOM. A three-stage fast preamplifier using radio frequency amplifier chips is designed to amplify the weak signal from the APD sensor. The bit synchronization signal acquisition and signal judgment modules are established to demodulate the detected pulse X-ray signals. The single photon pulse response and signal-to-noise ratio characteristic of the APD detector are tested. The pulse response width is only 25 ns. The BER characteristic with 211,709-bit pseudo-random sequence and the eye diagram of the XCOM system based on the developed APD detector are evaluated. Results show that the BER is zero at the data rate lower than 6 Mbps and reaches 1.98 × 10−4 at 10 Mbps under 55 kV anode voltage, 1.58 A filament current, and a 10 cm air channel. The designed APD detector increases the data rate of XCOM from the level of 1 Mbps to 10 Mbps, indicating its potential application for a higher data rate.

Linear-mode HgCdTe avalanche photodiode detectors for photon-counting applications (invited)

Linear-mode HgCdTe avalanche photodiode detectors for photon-counting applications (invited)

Development of Time- and Energy-Resolved Synchrotron-Radiation-Based Mössbauer Spectroscopy

Synchrotron-radiation based Mössbauer spectroscopy has become a useful technique capable for investigating various Mössbauer isotopes. For a typical experimental setup, the information associated with the pulse height (that is, energy) in an avalanche photodiode (APD) detector has not been used effectively. By using a system for simultaneous measurement system of time and energy associated with the APD signal, a system for the time- and energy-resolved Mössbauer spectroscopy has been developed. In this system, the pulse height information was converted to the time information through an amplitude-to-time converter applied to one of the divided signals from the APD. The corresponding time information was processed separately from another one of the divided signals. Both signals are recorded by a multi-channel scaler in an event-by-event data acquisition process. The velocity information from the Mössbauer transducer was also recorded as a tag for each signal event. Thus, the Mössbauer spectra with any time- and energy-window can be reconstructed after the data collection process. This system can be used for many purposes in time- and energy-resolved Mössbauer spectroscopy, and shows significant promise for use with other fast detectors and for various types of experiments.

Ability of strong-pulse illumination to hack self-differencing avalanche photodiode detectors in a high-speed quantum-key-distribution system

Implementation of high-speed quantum key distribution~(QKD) has become one of the major focus in the field, which produces high key-generation rate for applications. To achieve high-speed QKD, tailored techniques are developed and employed to quickly generate and detect quantum states. However, these techniques may introduce unique loopholes to compromise the security of QKD systems. In this paper, we investigate the loopholes of self-differencing~(SD) avalanche photodiode~(APD) detector, typically used for high-speed detection in a QKD system, and demonstrate experimental testing of SD APD detector under strong pulse illumination attack. This attack presents blinding stability and helps an eavesdropper to learn the secret key without introducing extra QBER. Based on this testing, we propose a set of criteria for protecting SD APD detectors from the strong pulse illumination attack.

4-bit DAC based 6.9Gb/s PAM-8 UOWC system using single-pixel mini-LED and digital pre-compensation.

Low-cost underwater wireless optical communication (UOWC) systems are attractive for high-speed connections among unmanned vehicles or devices in various underwater applications. Here we demonstrate a high-speed and low-cost UOWC system using a low-resolution digital to analog converter (DAC), a single-pixel mini-sized light-emitting diode (mini-LED), and digital pre-compensation (DPC). The enabled DPC scheme comprises digital pre-distortion (DPD), digital pre-emphasis (DPE), and digital resolution enhancer (DRE), which pre-compensate for mini-LED nonlinearity, the bandwidth limitation of the mini-LED and avalanche photodiode detector, and DAC resolution limitation, respectively. The simulation results show that the in-band signal-to-quantization noise ratio can be increased by 6.8 dB using DRE based on a 4-bit DAC. To further improve the system capacity, we tune the level of DPE in order to optimize the trade-off between the residual inter-symbol interference and signal-to-noise ratio. With the combination of optimized DPE and DRE, we obtain a 21.1% higher data rate compared with full DPE only and demonstrate the transmission of 6.9 Gb/s PAM-8 signal over a 2-m distance underwater based on a single-pixel mini-LED and 4-bit DAC. This paper reports a cost-effective UOWC system first using a low-resolution DAC and DPC, which offers a promising path toward low-cost underwater optical wireless networks.

Performance analysis of PPM FSOC system with APD detector considering atmospheric turbulence channel and fiber coupling

In a free-space optical communication system with an avalanche photodiode (APD) detector, the average bit error rate (ABER) is studied based on pulse position modulation (PPM) considering Gamma–Gamma atmospheric turbulence and fiber coupling efficiency (FCE). By analyzing the shot noise and thermal noise, the approximate analytical expression of the ABER of binary PPM is theoretically derived. Then, the approximate analytical expression of ABER union bound is derived for L-ary PPM. The FCE has a greater impact on the ABER of the APD detection system than it does on the PIN detection system. By adopting the adaptive optics technology, compared with the PIN detection system, the communication performance of the APD detection is greatly improved. Moreover, the optimal average APD gain of the APD detection system is strongly correlated to the detector temperature but weakly dependent on the atmospheric turbulence intensity, FCE, number of wavefront compensation terms, average received photon number, and receiving aperture size. By optimizing the receiving aperture and designing an accurate temperature control system, the communication performance of the APD detection system can be further improved.

Photon arrival sequence analysis of laser heterodyne signal with a Geiger-mode APD array

A heterodyne detection mechanism with Geiger-mode avalanche photodiode detector arrays is proposed for optical systems to achieve high sensitivity and good anti-interference. In contrast to the conventional processing method of photon arrival sequence, where the step of incoherent accumulation is canceled, the beat frequency is only obtained in this paper by the coherent superposition of phase-matched sequences, which can alleviate or even reduce the effect of the initial phase drift between the frames of the beat frequency signal. After phase matching of the beat signal, the power spectral density intensity of coherent superposition is greater than that of incoherent accumulation.

Efficient design and implementation of a distributed temperature sensor for field apparatus

The signal acquisition and processing ability of distributed temperature sensor based on Raman scattering (DTS-Raman) directly determines the accuracy, spatial resolution, temperature resolution, and other key indicators of the entire system. Real-time and effective processing is the key to ensuring the practicability of the field system. Owing to the long distance of the DTS system, the energy of the weak Raman scattering is further reduced when it reaches the avalanche photodiode detector because of fiber loss, which results in a decrease in the signal-to-noise ratio (SNR) and an increase in the temperature measurement error. To improve the SNR and ensure the accuracy of the demodulation temperature, the signal acquisition and processing module must be able to perform real-time and fast processing of large amounts of data. The existing processing structure based on the acquisition board requires a special personal computer and host processing software, which limits the application of DTS system. Therefore, a real-time acquisition and processing scheme based on field-programmable gate array is proposed in this paper. A set of cyclic shift register sequence (CSRS) with Ping-Pang functions is designed to improve the storage efficiency. According to the period of the laser source, the accumulated results stored in the CSRS and echo pulse data are accumulated and shifted in real time to realize circular utilization of storage resources. A DTS-Raman system prototype and test platform are built to verify the effects of the proposed scheme. The experimental results show that the scheme can realize a temperature accuracy within in real time and has a high resource utilization efficiency.

High-throughput nuclear resonance time domain interferometry using annular slits.

Nuclear resonance time domain interferometry (NR-TDI) is used to study the slow dynamics of liquids (that do not require Mössbauer isotopes) at atomic and molecular length scales. Here the TDI method of using a stationary two-line magnetized 57Fe foil as a source and a stationary single-line stainless steel foil analyzer is employed. The new technique of adding an annular slit in front of a single silicon avalanche photodiode detector enables a wide range of momentum transfers (1 to 100 nm-1 by varying the distance between the annular slits and sample) with a high count rate of up to 160 Hz with a Δq resolution of ±1.7 nm-1 at q = 14 nm-1. The sensitivity of this method in determining relaxation times is quantified and discussed. The Kohlrausch-Williams-Watts (KWW) model was used to extract relaxation times for glycerol. These relaxation times give insight into the dynamics of the electron density fluctuations of glycerol as a function of temperature and momentum transfers.

BER Performance of UWOC With APD Receiver in Wide Range Oceanic Turbulence

In this paper, the bit error rate (BER) performance of underwater wireless optical communication (UWOC) systems with on-off keying (OOK) modulation and an avalanche photodiode (APD) detector are analyzed in a wide range of oceanic turbulence environment, such as lognormal, Gamma, K, Weibull, and Exponentiated Weibull distributions. To achieving this goal, a new mathematical system model is established, which has not only considered the aggregated degrading phenomena, namely the absorption, scattering, and turbulence, but also been suitable for describing UWOC with APD receiver. Then the closed-form expressions of the BER performances of UWOC with APD for different turbulence distributions are derived in detail by subdividing the working situation into high and lower inter-symbol interference (ISI) scenarios. Theoretical analysis and computer simulations show that no matter what the ISI levels are, the derived closed-form expressions for system BER under various turbulences can approximate the accurate integral ones well. This is obviously helpful to the design and performance evaluation of UWOC systems.

Effect of the APD Receiver’s Tilted Angle on Channel Capacity for Underwater Wireless Optical Communications

This paper focuses on investigating the effect of the receiver’s tilted angle on the channel capacity of an underwater wireless optical communication (UWOC) system, in which an avalanche photodiode (APD) detector is adopted as the receiver. Under the non-negativity, peak power, and average power constraints, the lower bounds on the capacity of UWOC are derived in detail according to different average-to-peak power ratios. With modeling achieving the maximum of the lower bounds of the capacity as an optimization object, we prove that the proposed optimization issue is in fact a simple convex optimization about the tilted angle of the APD receiver, and then present related theoretical solution for it. Both theoretical analysis and simulation results show that by appropriately tilting the receiver, we can significantly enhance the final capacity performance of the UWOC with APD receiver.

Large Adaptive Optics Survey for Substellar Objects around Young, Nearby, Low-mass Stars with Robo-AO

Abstract We present results from the Large Adaptive optics Survey for Substellar Objects, where the goal is to directly image new substellar companions (&lt;70 M Jup) at wide orbital separations (≳50 au) around young (≲300 Myr), nearby (&lt;100 pc), low-mass (≈0.1–0.8 M ⊙ ) stars. We report on 427 young stars imaged in the visible (i′) and near-infrared (J or H ) simultaneously with Robo-AO on the Kitt Peak 2.1 m telescope and later the Maunakea University of Hawaii 2.2 m telescope. To undertake the observations, we commissioned a new infrared camera for Robo-AO that uses a low-noise high-speed SAPHIRA avalanche photodiode detector. We detected 121 companion candidates around 111 stars, of which 62 companions are physically associated based on Gaia DR2 parallaxes and proper motions, another 45 require follow-up observations to confirm physical association, and 14 are background objects. The companion separations range from 2 to 1101 au and reach contrast ratios of 7.7 mag in the near-infrared compared to the primary. The majority of confirmed and pending candidates are stellar companions, with ∼5 being potentially substellar and requiring follow-up observations for confirmation. We also detected a 43 ± 9 M Jup and an 81 ± 5 M Jup companion that were previously reported. We found 34 of our targets have acceleration measurements detected using Hipparcos–Gaia proper motions. Of those, 58 − 14 + 12 % of the 12 stars with imaged companion candidates have significant accelerations ( χ 2 &gt; 11.8 ), while only 23 − 6 + 11 % of the remaining 22 stars with no detected companion have significant accelerations. The significance of the acceleration decreases with increasing companion separation. These young accelerating low-mass stars with companions will eventually yield dynamical masses with future orbit monitoring.

Detector blinding attacks on counterfactual quantum key distribution

Counterfactual quantum key distribution protocols allow two sides to establish a common secret key using an insecure channel and authenticated public communication. As opposed to many other quantum key distribution protocols, part of the quantum state used to establish each bit never leaves the transmitting side, which hinders some attacks. We show how to adapt detector blinding attacks to this setting. In blinding attacks, gated avalanche photodiode detectors are disabled or forced to activate using bright light pulses. We present two attacks that use this ability to compromise the security of counterfactual quantum key distribution. The first is a general attack but technologically demanding. (The attacker must be able to reduce the channel loss by half.) The second attack could be deployed with easily accessible technology and works for implementations where single photon sources are approximated by attenuated coherent states. The attack is a combination of a photon number splitting attack and the first blinding attack which could be deployed with easily accessible technology. The proposed attacks show counterfactual quantum key distribution is vulnerable to detector blinding and that experimental implementations should include explicit countermeasures against it.