We find that photons from light emitting
diode injected with Poissonian electrons have super-Poissonian
statistics when the differential efficiency is larger than the
mean emission efficiency of the device. This result shows the
limitation of a conventional theory which describes the generation
of sub-Poissonian light in term of Poisson point process.

With the use of semiconductor
devices such as laser diode (LD) and light emitting diode (LED),
it is not difficult to generate sub-Poissonian light or nonclassical
light by injecting *quiet* electrons. Compared to the scheme
using nonlinear optical processes, the scheme using semiconductor
devices has the advantage of simple experimental configuration,
small energy consumption, and good controllability by changing
the structure of the device and the statistics of injection current.
However, there are some unsolved problems. Firstly, we do not
have an established quantum mechanical theory which describes
the process of generating sub-Poissonian light and we can not
predict the quantum state or density matrix of sub-Poissonian
light. Secondly, in the most of the experiment, the measured quantity
is restricted to the magnitude of the photon number fluctuations.
Other properties such as intensity correlation function and phase
fluctuations remain unexplored. Thirdly, it is not easy to generate
*weak* sub-Poissonian light over *broad* bandwidth using
conventional commercially-available LED. To the best of our knowledge,
the largest squeezing realized with LED was 3.1dB, the weakest
intensity was 7W, and the broadest measured bandwidth was 10MHz
[1]. The aim of our work is to experimentally investigate these
issues.

The Fano factor gives us a
convenient measure of the magnitude of number fluctuations. It
is defined by the variance normalized by that of Poisson distribution.
The variance of Poisson distribution is equal to its mean value,
therefore the Fano factor *W* is defined as *W=<n ^{2}>/<n>*.
In a simple point-process picture, the Fano factor of the output
photons W

*W_{PH}=*1

Where is the emission efficiency.
In this picture, the difference between the differential and mean
efficiency is not considered. According to eq.(1), the magnitude
of noise suppression is equal to when *W _{EL}*=0,
wheres

Fig.1 Experimental Arrangement. By switching from the resistor Rs tothe photodiode, sub-Poissonian and Poissonian noise power can be measured.

In our experiment, the noise
power for *W*EL 0 (sub-Poissonian mode operation) is normalized
by that for WEL1 (Poissonian mode operation). This normalization
method is used in the most of the previous experiments because
the change of experimental apparatus can be minimized. The experimental
setup is shown in Fig.1. In sub-Poissonian mode operation, a high-impedance
resister is used to suppress current fluctuations. In Poissonian
mode operation, a photodiode illuminated by white light (light
from low efficiency LED or electric bulb) is used as a stochastic
regulator. By just switching from the resistor Rs to the photodiode,
sub-Poissonian and Poissonian noise power can be measured.

Figure 2 shows the normalized noise power for various injection current levels. The normalization was performed after subtracting the amlifier noise. The type of LED used in this experiment was Hitachi model HLP40RD, which is a GaAlAs LED with a single heterojunction structure. Starting from the bottom curve, the photo-current detected by PD2 was 500A, 50A, and 5A respectively. By extrapolating the trace, the nomalized noise power at zero frequency is estimated and the results are shown in Table 1. If eq.(1) is correct, the normalized value should be equal to

_{PD} | |||

Table 1. The values of normalized noise power and theoretical predictions

(1)/1=1. However, in all cases,
experimental values were smaller than 1. This means that the description
in terms of photonic stochastic point process is insufficient
in contrast to the argument by Edwards *et al*.[4].

In the point process description
where photons and electrons are treated as classical particles,
it is not easy to intuitively consider the effect of differential
efficiency. Thus we here describe the situation in term of small
signal transfer as is shown in Fig.3. The horizontal axis represents
the input current and the vertical axis represents the output.
In our experiment, the input-output characteristic of LED is not
linear. In this case, the mean efficiency that is expressed by
the slope of a straight line drawn from the origin is smaller
than the differential efficiency * _{d}* that is expressed
by the slope of a tangential line. Then, for an average input
current of

*W_{OUT}=*1

For *W _{IN}*
=1,

In order to more directly
check the validity of eq.(2), we artificially decreased the efficiency
by inserting a pinhole between LED and PD2 and measured noise
power. Results are shown in Fig.4. The injection currents were
in Poissonian mode, and their levels were adjusted so that the
photo-current detected by PD2 became the same. Without a pinhole,
the mean and differential efficiencies were 16.1% and 19.5%, respectively.
With a pinhole they were 4.3% and 4.7%. Using eq.(2), the output
Fano factors are calculated to be 1.058 for the former and 1.005
for the latter. According to eq.(1) they should be the same. As
is clearly seen in Fig.4, the noise power for two cases are different.
The upper trace shows the ratio of two noise powers and its magnitude
coincides well with the theoretical prediction of 1.05 (shown
by arrow).

We experimentally investigated
the effect of differential efficiency on the generation of sub-Poissonian
light by LED. We have found that the simple point process description
is insufficient when the mean emission efficiency differs from
the differential efficiency. Experimental results agreed well
with a new formula. This suggests a new possible scheme for generation
of sub-Poissonian light in which differential efficiency is made
to be smaller than mean efficiency.

[1] T. Hirano and T. Kuga, IEEE J. QE., 31, 2236 (1995); G. Shinozaki, T. Hirano, T. Kuga and M. Yamanishi, CLEO/Pacific Rim '95, WG3 (July 1995).

[2] M.C. Teich and B.E.A.
Saleh, Opt. Lett. __7__, 365 (1982).

[3] Akira Shimizu, Ouyou Butsuri
(in Japanese), __62__, p.881-888, (1993).

[4] P.J. Edwards and G.H.
Pollard, Phys. Rev. Lett., __69__, 1757 (1992).

[5] Hiroshi Fujisaki and Akira Shimizu, CLEO/Pacific Rim'95, PD1.7.