In order to asses free space optical links, using a Cassegrain telescope, for both classical (i.e. coherent and direct
detection) and quantum (coherent, Dolinar-Kennedy system) several scenarios were analyzed. This analysis was
conducted through a software tool that uses some specific parameters imposed by the environmental condition and
hardware used, such as: pointing and tracking errors, turbulence, modulations schemes, and visibility, among others; in
order to obtain the general performance parameters of the simulated link, such as: bit error rate and the photons number.
The software developed allows the user to produce useful way to assess the possible success of the implementation of
the free space optical link, or a way to identify phenomena that might suggest hardware adjustments to improve the
overall performance of the system. Results allow the revision of the expected link performance for a quantum key
distribution system; although it is possible to use it for general optical quantum communications systems.
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.
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.