Device noise measurement system

Details Device noise measurement system Device noise measurement system

1999-04-15

2001-11-20

Font, Frank G. (Department: 2877)

Radiant energy

Photocells; circuits and apparatus

Photocell controlled circuit

C250S2140LS, C356S226000, C356S222000

Reexamination Certificate

active

06320179

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to a system which may be used to measure the noise levels of laser devices, including continuous wave and pulsed lasers and, in particular, semiconductor laser diode devices such as those used in telecommunications. The system may be used to measure relative intensity noise (RIN) levels. RIN measurements are of particular importance in the field of fibre optic communications.
Spurious, random laser omissions cause intensity fluctuations in the output from laser sources. These fluctuations form the noise floor which varies with frequency and peaks at the relaxation resonance of the laser. The location of the relaxation resonance, intensity noise peak, is directly related to the maximum modulation rate of the laser. Interaction between the optical field in the laser and the injected-electron density, due to the bias current, causes the relaxation resonance to vary as a function of the bias current.
An important parameter in evaluating both laser and system performance for broadband digital and analogue systems is the relative intensity noise (RIN). RIN is the ratio of the mean-squared-intensity-fluctuation spectral density of the optical power output to the square of the average optical power. Some known measurement systems for measuring RIN are based around the use of a photodetector diode, a high gain amplifier and an electrical spectrum analyser. The optical laser energy is transferred to electrical energy by the photodetector diode and is amplified to enable low level intensity noise to be measured by the spectrum analyser.
Existing systems for measuring RIN of a laser are described in the following references; U.S. Pat. No. 5,534,996 and M. Puleo, “Ultra-high sensitivity technique for characterisation of laser diode intensity noise in the GHz range”, CSELT Technical Reports, vol. 17, no. 3, June 1989, Italy pp 199-200.
One of the disadvantages of existing systems is that the frequency response and linearity of the photodetector, amplifier and spectrum analyser must be well characterised to obtain accurate measurements across a useful, wide frequency range, typically between 20 MHz and 20 GHz. Furthermore, factors such as mismatch losses, detector capacitance and spectrum analyser amplitude errors also need to be accounted for and this makes system calibration complex and time consuming.
JP A 02 189 474 describes an apparatus for measuring the noise of an optical receiver, as opposed to an optical source, and is general background to the present invention.
The present invention relates to a system and method for measuring noise levels of laser devices. In particular, the system may be used to measure relative intensity noise levels for semiconductor laser diodes such as those used in telecommunications. The system design is such that system calibration can be calculated simply and frequently, reducing systematic errors and calibration costs. Using this system, RIN measurements with an accuracy of less than 1 dB may be achieved across a frequency range of between 10-20 MHz and 20 GHz.
SUMMARY OF THE INVENTION
According to one aspect of the invention, a system for measuring one or more noise contributions to an optical output signal, OPT
DUT
, from a device comprises;
means for generating a broadband, substantially shot noise limited optical reference signal, OPT
REF
,
detection means for receiving the optical signals, OPT
DUT
and OPT
REF
, and converting said optical signals into equivalent electrical signals, ELEC
DUT
and ELEC
REF
respectively, each comprising an ac component and a dc component, wherein the optical signals OPT
REF
and OPT
DUT
are such that the dc components of the electrical signals, ELEC
DUT
and ELEC
REF
, are substantially equal,
means for separating the ac components and the dc components of the electrical signals, ELEC
DUT
and ELEC
REF
,
means for measuring the dc components of the electrical signals, ELEC
DUT
and ELEC
REF
,
means for amplifying the ac components of the electrical signals and
means for measuring the amplified ac components of the electrical signals at a plurality of frequencies,
whereby the measurement of at least one of the ac components or dc components of the electrical signals, ELEC
DUT
and ELEC
REF
, provides an indication of one or more of the noise contributions to the optical output signal, OPT
DUT
.
In one embodiment of the invention, the system may comprise means for generating a substantially shot noise limited optical reference signal, OPT
REF
, having a linewidth less than a lower frequency limit and wherein the system also comprises means for measuring the amplified ac components of the electrical signals at a plurality of frequencies above the lower frequency limit.
In a preferred embodiment, the system also comprises first optical isolation means through which the optical signal, OPT
DUT
, is transmitted so as to minimise reflection of the optical signal OPT
DUT
. In a further preferred embodiment the system also comprises second optical isolation means through which the optical signal, OPT
REF
, is transmitted so as to minimise reflection of the optical signal OPT
REF
.
For example, the system may comprise at least one of a first free space optical isolator component and a first fibre optical isolator component through which the optical signal, OPT
DUT
, is transmitted and at least one of a second free space optical isolator component and a second fibre optical isolator component through which the optical signal, OPT
REF
, is transmitted.
The means for generating an optical reference signal, OPT
REF
, may comprise a reference laser, for example a diode pumped Er doped glass ring laser. Alternatively, the system may comprise an Erbium doped fibre amplifier and a narrowband filter so as to yield a flat noise spectrum up to a frequency limit determined by the narrowband filter.
The means for generating an optical reference signal may further comprise attenuation means for attenuating the optical signal output from the reference laser, OPT
REF
. The attenuation means may also be used to optically attenuate the optical signal output from the device, OPT
DUT
. The attenuation means may be an optical attenuator capable of providing an attenuation of between 0 and 60 dB.
The detection means may be a photodetector and the means for measuring the amplified ac component of the electrical signal may be a spectrum analyser.
The means for amplifying the ac components of the electrical signals may be an amplifier capable of providing an amplification of between 10 dB and 50 dB, and more typically between 30 dB and 50 dB. The means for measuring the dc components of the electrical signals may be a voltmeter.
According to another aspect of the invention, a method for determining the device noise power contribution, N
L
, to the total noise power, N
T
, of an optical signal, OPT
DUT
, output from a device, having a wavelength &lgr;, using apparatus contributing a thermal noise power, N
TH
, to the total noise power, N
T
, said total noise power also comprising a shot noise power contribution, N
Q
, comprises the steps of;
(i) generating a broadband, substantially shot noise limited optical reference signal, OPT
REF
,
(ii) detecting the optical signals, OPT
DUT
and OPT
REF
, and converting said optical signals into equivalent electrical signals, ELEC
DUT
and ELEC
REF
respectively, using detection means, wherein ELEC
DUT
and ELEC
REF
each comprise an ac component and a dc component, wherein the dc components of the electrical signals, ELEC
DUT
and ELEC
REF
, are i
dc(DUT)
and i
dc(REF)
respectively, and varying the optical reference signal, OPT
REF
, so that i
dc(DUT)
and i
dc(REF)
are substantially equal,
(iii) separating the ac component and the dc component of the electrical signals, ELEC
DUT
and ELEC
REF
,
(iv) measuring one or more of the dc components, i
dc(DUT)
and i
dc(REF)
of the electrical signals, ELEC
DUT
and ELEC
REF
,
(vi) amplifying one or more of the ac components of the electrical signals, ELEC
DUT
and ELEC
REF
,
(vii) measuring one or more of the am

Affiliated with

Also associated with

Rating

Device noise measurement system does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Device noise measurement system, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Device noise measurement system will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2572357