CLASIFICACIÓN Y MANEJO DE LA BASURA:

The maximum signal bandwidths of most (directly-modulated) semiconductor laser diodes and optical detectors are in the region of 1 GHz, which limits the minimum effective optical pulse-length to about 1 metre [= pulse duration o f 3 ns]. As such, pulse position modulation sensors are generally limited to extrinsic rather than intrinsic modulation mechanisms with only a few implementation options available. Examples of PPM sensors are laser range-finders, tachom eters based on double-pulse transm issive or reflective operation and some hybrid (ie. fibre optically-pow ered electrical) sensors. One intrinsic PPM sensor which has been reported [Johnson and Ulrich, 1978] is a strain sensor based on the return time delay o f an optical pulse transmitted down and reflected back from the end of a sensing fibre; however, as the return time delay changes typically only 10 fs per ^istrain per metre of sensing fibre, the sensing length needs to be over 100m for only moderate sensitivity which is impractical for most applications.

4 .5 Pulse-W idth M odulation (PWM).

In this scheme the sensor modulates pulse width in response to the measurand. Pulse W idth Modulation (PWM) sensors demonstrated to date include temperature sensors based on infra-red fluorescent decay rates in inorganic phosphors [Grattan and Palmer, 1985] and some hybrid sensors [Spooncer et al., 1987]. It is hard to arrive at a

generalised passive PWM sensor or system architecture for useful m easurand interm ediaries like linear or rotary displacement which excludes this scheme from further analysis.

4 .6 Pulse-Frequency M odulation (PFM ).

The options for Pulse Frequency Modulation (PFM) are much more varied than for PPM and PWM, as interferometric and micro-resonator principles can be exploited. Interferometric operation is usually by the FMCW technique (see Section 2.4.2.1.2) in which the sensing interferometers or polarimeters are set with unique imbalances so that their FM outputs are frequency division m ultiplexed; dem odulation can be by frequency-discrimination or better still, by pulse counting (see Section 4.7).

O f the micro-resonator sensors, the most simple to integrate into systems are the optically-excited versions [Stokes et al., 1988] which require only CW coherent radiation above a threshold power level in order to produce a high-frequency deviation lead-insensitive sub-carrier FM output. Optical-excitation is usually established by placing a semi-transparent mirror in front of the silicon bridge to form a Fabry-Perot cavity; this introduces a position dependency into the thermal absorption of the bridge structure which, when the operating point is adjusted, establishes self-oscillation provided the optical power is above a threshold level [W] given by [Zhang et al.,

1989]:

p > A _ “ F.X

(4.6) where:

F is the finesse of the Fabry-Perot cavity (typically 3).

X is the optical response of the bridge (deflection for a given irradiated power); typically 0.001 m/W (l|xm/mW).

This threshold power is typically in the region of 200 |xW, so for a star network with a coupler excess loss of 0.5 dB and splice losses of 0.2 dB (ie. a drop loss of 12.5 logjo(Njj) dB, see Section 1.4.4.1) the system capacity will be limited to « 22 sensors if intrinsic safety is required (P^ax*^ 10 mW [Hills, 1990] ) and approximately « 250 sensors if not (assuming P ^ ^ = 200 mW, high-power Spectra Diode Laboratories CW laser with fibre pigtail).

4 .7 P u ls e -C o u n tin g S c h e m e s.

The most obvious example of pulse-counting operation is simple event counting (eg. batch counting) using binary (ON/OFF) sensors. A more sophisticated example is the application to interferometers operating in FM CW -mode proposed by [Ohba et al., 1989]. Ohba noted that in operating an interferom eter using an optical frequency (sawtooth-) ramped source (as in the FMCW technique [Franks et al., 1985]), the number of fringes N. swept over the ramp period is proportional to both the optical path imbalance OPD- [m] of the interferometer and the maximum optical frequency sweep Af^jpt [Hz] of the source and is independent of the linearity of the optical frequency ramp (provided that the frequency change remains monotonie over the ramp period):

N. = (4.7)

c

The measurement of a parameter causing change in the path imbalance o f a sensing interferom eter (eg. temperature or strain) can therefore be accurately made using a directly-m odulated or temperature-modulated sem i-conductor laser since the only unknown, the optical frequency range of the source, can be eliminated by dividing the fringe count in the sensing interferom eter by the fringe count Nq m easured in a reference interferometer (in a stabilised environment) having a fixed path imbalance OPD„:

(4.8)

N„ OPD„

The measurement resolution of this system (ie. the change in optical path difference AOPD [m] to cause N. to increment or decrement by one fringe) is given by:

A 0PD = ^ = : ^ . ^ . L . . 5 x (4.9)

2jt

where:

S* is the phase sensitivity of the sensing fibre to measurand x (typically, for a monomode fibre, the temperature phase sensitivity S j is » 100 rad.m and the strain phase sensitivity « 10 rad.m '^|xstrain‘^).

Lg is the sensing length [m].

The maximum continuous frequency range of GaAlAs laser diodes (X=830 nm) is of order 50 GHz for both direct (current) modulation and thermal modulation giving the following estimated measurement resolutions:

Ls 5T/®C 6e/}i strain

4.5 km 0.1 1

450 m 1 10

45 m 10 100

Table 4.1. Estimated measurement resolution of a passive sensing system using pulse-counting.

The system will clearly offer only moderate measurement sensitivity for practical sensing lengths but has the advantage of allowing frequency-division m ultiplexed operation by the use of different optical path imbalances for each interferometer in the netw ork (see Fig. 4.5). As noted in Section 2.4.2.1 .2 (eq n .(2 .5 5 )), the beat frequencies fy. of the interferometer outputs in an FMCW system are given by:

T .c (4.10)

where T^ is the period of the ramp modulation [s] and L. the path imbalance [m] in the i**' interferometer. FDM operation requires that the fringe counts of all sensors in the network are contained within unique ranges and do not overlap at any time. Since the maximum fringe count N p ^ is set by the source coherence length [m] by way of:

(4.11)

the maximum number of sensors is limited by N ^ ^ .D < Np^^j^ where D is the dynamic range (or maximum number of measurement states) of the sensors. Typically, single-longitudinal mode GaAlAs laser diodes have a power-bandwidth product of 100 MHz-mW so that « 30 m at 10 mW. Np^^j^is then « 5,000 so that approximately 50 sensors (max) can be accommodated with a dynamic range of 100.

This system shows excellent potential for multi-level measurement applications due to the simplicity of the sensors and processing electronics and insensitivity to source non- linearities (bar mode-hopping which will need to be avoided by temperature control and optical isolation).

4 .8 In v estig a tio n into the optical pow er e ffic ie n c ie s o f various pulse- m o d u la tio n fo rm a ts in in te n sity -m o d u la te d o p tic a l fib re se n sin g sy stem s.

This section will compare the following pulse-modulated systems in terms of estimated maximum sensor capacities in the most common situation where the dominant system noise source is signal-independent receiver noise (eg. thermal or amplifier noise):

1. Binary Pulse Code Modulation (PCM). 2. M-ary PCM.

3. Pulse Amplitude Modulation (PAM). 4. Pulse Position Modulation (PPM).

5. Amplitude Modulation of Frequency Division Multiplexed sub-carriers (AM/FDM).

6. Frequency Modulation of FDM sub-carriers (FM/FDM).

The prim ary aim of the analysis is to compare each scheme for the same system requirem ent and under identical conditions rather than accurately identify the multiplexing limit which can practically be obtained from each system using specific com ponents. The analysis will be based on the follow ing com m on system requirements: