We describe a homodyne optical Costas loop receiver intended to detect weak coherent states with diffused phase and suppressed carrier phase modulation. In order to get the information contained in the quadrature components of the optical field, we implement an 8-port receiver operating at 1550 nm, based on the manipulation of the state of polarization of both the local oscillator and the data signal. Employing binary phase-shift keying, we make measurements in the time and frequency domain of the quantum noise and bit error rate using an optimum loop filter, and compare the performance of our receiver against the standard quantum limit for the simultaneous quadrature detection, considering both ideal conditions and the overall efficiency of our set up.
We present an experimental 8-port Balanced Homodyne Detector at 1550 nm wavelength, operating in free space,
implemented with polarization devices to produce a circularly polarized local oscillator, splitting its In-Phase and
Quadrature components to beat separately with the weak coherent incoming signal. This allows the simultaneous
measurements of the 2 quadratures at the price of an additional noise due to the vacuum fields that leak via the unused
ports, resulting in a modified Husimi function for joint probability distribution for the quadrature components. These
schemes require the proper optical phase synchronization between the local oscillator and the incoming field, which
constitutes a challenge for weak coherent state reception. To achieve this we designed and implemented an optical
Costas loop; the feedback loop (especially the loop filter) which is a result of the optimal design has an impact on the
mutual information between transmitter and receiver, being this parameter a condition to generate the cryptographic key.
We present experimental and theoretical results on the performance of the mutual information between the transmitter
and the receiver due the phase error for different photon numbers.
We present the evaluation of wireless optical communication systems through laboratory recreation of atmospheric
turbulence, and analyze the scope of this form of experimental validation in various optical communication systems.
Weak coherent states (WCS) are being extensively employed in quantum communications and cryptography at
telecommunications wavelengths. For these low-photon-number applications, simultaneous field quadrature
measurements are frequently required, such as in the detection of multilevel modulations in the communications scenario
or in cryptographic applications employing continuous variables. For this task multiport balanced homodyne detection
(BHD) structures are employed, based on the splitting of the received field into its (non-commutating) in-phase (I) and
quadrature (Q) components and their separate beating with a local oscillator (LO) in two BHD. This allows the
simultaneous measurements of the 2 quadratures at the price of an additional noise due to the vacuum fields that leak via
the unused ports. These schemes require the proper optical phase synchronization between the LO and the incoming
field, which constitutes a challenge for WCS reception, especially for suppressed carrier modulations that are required
for power economy. For this task, a Costas loop is implemented for low photon number WCS, with the design of an
optimum feedback scheme considering the phase diffusion of WCS generated by semiconductor lasers. We
implemented an optical Costas loop at 1550 nm based on polarization splitting of the laser field to detect I and Q
quadratures simultaneously. We present results on the performance in phase error and bit error rate and compare with
corresponding quantum limit.
Optical direct detection usually operates far above the quantum limit, due to the high thermal noise level of PIN
photodiodes. For signal energy at the quantum level, the thermal effects in photon counters are also a strong limitation.
The optical amplification or the heterodyne detection of the 2 quadratures of the field, widely used in high bit rate and
long haul optical systems, overcome this limitation at the expense of a minimum 3db noise figure. By allowing a noise
free mixing gain, as well as single quadrature measurements, the balanced homodyne receiver is allowed to reach
quantum noise limited operation.
The aim of this paper is to review the different quantum receiver implementations and to compare the minimum signal
energy required to achieve a given bit error rate, or a given bit erasure rate, in high bit rate communications and quantum
communications. Application to quantum cryptography will be also addressed.
We present an application of coherent homodyne detection to the problem of low photon number communications and
cryptography. As the coherent demodulation of an optical field requires the measurement of its (non commutating) inphase
and quadrature components, we present the structure and operation of an 8-port optical hybrid comprising 2
balanced homodyne detection structures, for the simultaneous measurement of the 2 quadratures. We analyze this
receiver operating with a strong local oscillator field, when the received field is in weak coherent states, with digital
phase modulation: we obtain the homodyne statistics and the uncertainty product in the presence of vacuum noises from
the input signal port and unused ports and discuss the increase in uncertainty due to the simultaneous measurements of
the quadratures. We obtain the signal to noise ratio as well of the bit error rate performance for binary phase shift keying
and discuss the departure from the standard quantum limit.
The optical wireless communications systems are an important alternative for providing high bandwidth over short or
medium range links. Although, most of such links are currently, based in no-coherent schemes, there exist the optical
coherent techniques offering a greater sensitivity and selectivity at the receiver stage and the possibility of impairments
compensation using high-speed post-detection DSP algorithms. We present the a scheme to demonstrate the use of the
optical coherent technique in wireless communications for a last-mile application under turbulence, in the laboratory
This paper presents the principles and experimental results of an optical fiber QKD system operating at 1550 nm, and
using the BB84 protocol with QPSK signals. Our experimental setup consists of a time-multiplexed super-homodyne
configuration using P.I.N detectors in a differential scheme as an alternative to avalanche photon counting. Transmission
over 11km of optical fiber has been done using this detection scheme and major relevant characteristics such as noise,
quantum efficiency and bit error rate (BER) are reported.
Coherent reception techniques are receiving much attention in fiber optic communications due to their numerous advantages related to increased repeater spacing and to the capability of densely spaced wavelength division multiplexing.