九州大学大学院総合理工学府報告第 42 巻 第 1 号 19-26 頁 令和 2 年 9 月

Engineering Sciences Reports, Kyushu University
Vol. 42, No. 1, pp. 19-26, SEP, 2020

Amplifier-assisted Waveguide Sensing toward Breath-gas
Analysis
Wenying LI*1,† Yu HAN*1 Zanhui CHEN*1Leiyun WANG*1
Haisong JIANG*2 and Kiichi HAMAMOTO*2
E-mail of corresponding author: ri.uenin.271@s.kyushu-u.ac.jp†(Received July 31, 2020, accepted August 7, 2020)

We have proposed compact breath sensing device by using optical waveguide. As there isan inevitable huge loss in the waveguide, we have proposed and demonstrated opticalamplifier-assisted scheme so as to compensate the waveguide propagation loss. One important
issue of the amplifier-assisted scheme is the amplifier noise at sensing light wavelength. The
significant accumulated amplifier noise may surpass the sensing light intensity and prevent
ppm-order gas sensing. In this work, we calculated the necessary light intensity of the injected
light into the fiber by taking account of the influence of amplifier noise. In addition, we haveverified the proposed condition in experiment. As a result, the actual 3% CO2 was successfully
detected.
Key words: Breath sensing, Optical waveguide, Amplifier-assisted cavity ring-down spectroscopy

1. Introduction
Population aging problem has become one of
the significant social problem. Because of thisproblem, the demand of daily and easy healthcheck system for elder people is considered to
be important 1). Especially home-based healthmonitor is desired 2). Breath sensing 3-5) isavailable for the daily and easy health checkbecause exhaled breath contains a lot kinds ofdisease markers 6, 7). People only needbreathing-out toward the sensor. The density ofeach disease marker in breath is analyzed at
the time.
For home-use, breath sensor is desired torealize hand-held size, several diseases
detection at one time, and quick response
within a few seconds. Up to now, several typesof breath sensors 8-19) have been researched.
Among them, sensor based on infrared*1 Department of Applied Science for Electronics
and Materials, Graduate student*2 Department of Applied Science for Electronics
and Materials

absorption spectroscopy 20) has the possibility tosense several kinds of gases at the same time 2123). This is because each gas has its own eigen
absorption peak in wavelength 24). For sensing
ppm-order gas component in breath by using
infrared absorption, an extremely long opticalsensing path of several km is needed forsufficient light absorption (typically 3 dB)22). To
achieve the effectively long length sensing path(corresponding km order), CRDS (cavity ringdown spectroscopy) 25) has been proposed. In
CRDS system, km length sensing path isrealized by using internal reflection at thecavity-edges. In case when the mirror
reflectivity is set to be high to have 1,000 times
reflection with a meter-long cavity for instance,
the effective length may exceed km order 26).
One issue of the CRDS system as home-use
sensor is its large size.
We have proposed breath sensor utilizing
optical waveguide as the optical path forinfrared absorption based on CRDS. This isbecause waveguide is capable for integrating
several meter optical paths in a compact area

20

Amplifier-assisted waveguide sensing toward breath-gas analysis(1 cm2) 27-29). With sensing light propagating inwaveguide for more than 1,000 times, thecorresponding sensing path length is available
for ppm-order breath sensing. One problem isthe waveguide propagation loss. Huge amountof loss decreases the sensing light intensity. As
most of the sensing light intensity is reduced
due to the propagation loss, ppm-order breath
sensing becomes hard to be realized. As asolution, we have proposed amplifier-assisted
CRDS 30) to compensate the propagation loss.
For
hand-held
breathsensor,SOA(semiconductor optical amplifier) is a candidate
due to its capability of integration. To verify theeffectiveness of the amplifier-assisted CRDS,
we use an EDFA (Erbium-doped fiber
amplifier) instead of the SOA inside CRDS
system in this work. When EDFA is at a highpumping
condition,ASE(amplified
spontaneous emission) loops inside CRDS
system and be amplified in the EDFA. Then, it
starts self-lasing at a wavelength different fromsensing light. Once self-lasing happens,sensing light looses the gain from EDFA. Thisis because most of the gain is attributed tolasing wavelength. We have proposed
polarization direction control scheme 31) tosuspend self-lasing by weakening thecoherency condition. The coherency is afundamental requirement of making oscillationin cavity (namely, ring-cavity here) in general.
The result showed that the self-lasing intensity
was suspended, and the gain was improved to
24 dB from 14 dB at the sensing lightwavelength.
Whereas the self-lasing issue has been
improved, there is still an important issue inthe amplifier-assisted CRDS system. The
amplifier noise which exists at sensing lightwavelength is hardly eliminated. In ppm-ordergas sensing, the amplifier noise loops inside thesystem with sensing light for more than 1,000
times. The significant accumulated noise
intensity may surpass the sensing lightintensity and prevent ppm-order breath
sensing. Hence, the accumulated noise
intensity influences the sensing ability of
amplifier-assisted CRDS system directly. In
this work, to evaluate the lowest gas sensing
concentration of amplifier-assisted waveguide
CRDS, we calculated the accumulated
amplifier noise intensity at ppm-order gas
sensing condition. Then we estimated thenecessary injection light intensity for ppmorder gas sensing under the influence of theamplifier noise. The estimated results showed
several kinds of ppm-order gases (CO2, CH4,
NH3, and CH3COCH3) are sensing-available byusing the injection light intensity within 10
mW. Meanwhile, the actual 3% CO2 sensing
was also demonstrated experimentally toconfirm the effectiveness of this proposal shownin this paper.

2. CRDS system for gas sensing
2.1 Principle of CRDS
In general, a volatile gas (target gas) has its
own eigen absorption wavelength at infrared
band 32). Table 1 summarized the breathcontent in exhaled breath, their absorption
wavelength λ [μm], absorption cross-section σ[cm2], concentration N [ppm], and the relateddisease 33-37).
Table 1 Disease markers in human breath
Breathcontent

λ

N

σ

[μm]

[ppm]

CH4

1.65

2-10

[cm2]
1.6×10-20

NH3

1.51

0.5-5

CH3CO
CH3

1.68

CO

1.57

1.73.73.35.4

2.5×10-20
3.6×10-22
2.2×10-23

RelateddiseaseIntestinediseaseLiverdiseaseDiabetes
Diabetes

To detect the target gas concentration in
exhaled breath, km-order length of sensing
path is needed 22). One attractive technique toreduce this km-order length down to below 1 m
is known as CRDS method. Figure 1 shows theschematic configuration of the general CRDS
system. The gas-cell has high reflection mirrors
at the both ends of the gas-cell. The highreflection mirrors are used to reflect thesensing light inside the gas-cell. When thereflection happens for more than 1,000 times in
a 1 m length gas-cell for instance, thecorresponding sensing path length reaches tothe effective length of km-order. During eachreflection, slight portion of the not-reflected(therefore transmitted) light leaks out from thegas-cell through the high reflection mirror 38).
The measurement is carried to detect this leakout light intensity. The example measurement
result (light intensity as a function of time) isshown in the right-hand side of Fig. 1. The lightintensity decreases because of the leak-out

令和 2 年度

九州大学大学院総合理工学報告

light from the mirror at the gas-cell ends. Thissituation happens even for “no-gas in gas-cell”
situation. In the condition of “gas in gas-cell”,
the light intensity decreases faster than thecase of “no-gas” due to the gas absorption. Here,the cavity ring-down time 39) τ and τgas aredefined as follows: the time that light intensity
decreases to the criteria level (normally 1/e)
(black dash-line in Fig. 1). And τ and τgas are thecavity ring-down times for “non-gas in gas-cell”
and “gas in gas-cell” situation, respectively.
The target gas concentration N [ppm] isestimated by using the cavity ring-down times
as 40, 41):

N =( τ

1

gas

-

1

τ

)×

RT
σc ×P×NA×10-9(1)c [cm/μs] is light speed. Equation (1) shows that

Injected
pulse

Light intensity

gas concentration is detected by measuring thesensing time in CRDS system. Based on theanalysis above, CRDS system is capable for
ppm-order gas sensing.
Leak-out light

Leak-out
light

τ

Time

Light intensity

High reflection
mirrors
Leak-out
Injected
light
pulse

Sample gas

Leak-out lightGas absorption
τgas
Time

Fig. 1 Illustration of general CRDS (cavity-ring-downsystem).

The problem of general CRDS as hand-held
sensor is its large gas-cell size. To realize handheld sensor, we have proposed high-mesa
waveguide 42) as the optical sensing pathinstead of gas-cell. This is because waveguide
has a potential of integrating a long optical
path in a compact area. When sensing lightpropagates in the high-mesa waveguide, aportion of light goes out of the waveguide. This
portion of light intensity is used for gas
absorption. Gas absorption happens by using
waveguide as the sensing path. Hence,
waveguide with high-reflection films on both
facets is suitable in CRDS instead of general
gas-cell for hand-held sensor.

第 42 巻 第 1 号

21

2.2 Amplifier-assisted waveguide CRDS
In waveguide CRDS, waveguide has highreflection films on both facets. Sensing light isreflected rapidly inside the waveguide. One
problem is that, waveguide has propagation
loss. Most of the injected sensing light isreduced due to the propagation loss. For
instance, a 1 m length waveguide has 2 dB
propagation loss. When light reflecting inside
the waveguide exceeds 1,000 times for ppmorder gas sensing, the accumulated loss may
reach to 2,000 dB. Breath sensing is impossible
with this huge loss. To solve this problem, we
have proposed amplifier-assisted CRDS. In
amplifier-assisted CRDS, waveguide has nohigh-reflection film. The sensing light at theoutbound of the waveguide is guided to an
optical amplifier to compensate the waveguide
propagation loss. After amplification, thesensing light is leaded to the inbound of the
waveguide. This waveguide configuration;
however; enclose the sensing light into a loop
without leak-out light. It is impossible;
therefore; to monitor the leak-out lightintensity decrease as CRDS. As a solution,
optical couplers are used to connect the output
port of the amplifier and the inbound of the
waveguide. The experimental set-up of
amplifier-assisted CRDS is shown in Fig. 2 (a).
In this experimental set-up, an EDFA (Erbiumdoped fiber amplifier) is set inside the CRDS todemonstrate the effectiveness of the amplifierassisted CRDS. Two optical couplers are used
to guide the sensing light. The injected sensing
light-pulse is guided to the inbound of the
waveguide via coupler 1. The light coming out
from the outbound of the waveguide is
amplified by EDFA. Then, most of the sensing
light is leaded back to the inbound of the
waveguide via coupler 2 and 1. A portion of“leak-out light” is monitored via one output
branch of the coupler 2. The amount of leak-out
light is controlled by the branch ratio of coupler
2.
In this experiment, sensing light is at 1572
nm wavelength. This wavelength is theabsorption wavelength of CO2, which is a major
content in human breath. Figure 2 (b) are theamplifier-assisted
CRDS
results(lightintensity as a function of time when theinjected light intensity was 1 mW). The orange
pulse-trains correspond to the result of “before
amplification” situation. The blue pulse-trains

Amplifier-assisted waveguide sensing toward breath-gas analysis

correspond to the result of “after amplification”
situation.
Without
amplification,
lightintensity decreased immediately. It only loopedin waveguide for 3 times. When EDFA provided
5 dB gain and compensated part of the
propagation loss, the looping time increased to
16 from only a few. These results lead to thesensing path length extending more than 5times than that without amplification.
Several meters

Coupler 1
LD@1572nm
Coupler 2

0.8

0.4
0.2
2

Sensing
light

-40

400mA
300mA
200mA
100mA

-60
-80
1530

1550

1570

1590

1610 1630

Fig. 3 Optical spectrum of amplifier-assisted CRDS at
different EDFA pumping condition.

After(gain=5 dB)

0.6

Lasinglight

Wavelength [nm]Before

0

-20

EDFA
(a)

1.0

0

0

4
6
Time [μs]
(b)

8

10

Fig. 2 The experimental set-up of amplifier-assisted CRDS
and its results. (a) Experimental set-up, and (b) experimentresults before and after amplification.

3. Issues in amplifier-assisted CRDS
Amplifier is used in waveguide CRDS tocompensate the waveguide propagation loss. As
amplifier gain increasing, the sensing light
loops in waveguide for more times. The
corresponding sensing path length increases.
Amplifier inside CRDS; however; cause theself-lasing issue and the amplifier noise issue.
In this session, we will discuss about these twoissues and their solutions.

3.1 Self-lasing issue and solution
Amplifier is used inside a closed loop as
shown in Fig. 2 (a). This configuration;
however; may result in self-lasing when EDFA
is at high pumping condition 43). Figure 3 shows
the optical spectrum of the amplifier-assisted
CRDS under different pumping current of
EDFA. Sensing light wavelength was at 1572
nm. As shown in the figure, self-lasing
happened at 1564 nm wavelength. It seems
that the self-lasing happened when injection
current of the pumping LD in the EDFA wasOnce this self-lasing happens, sensing lightis not amplified sufficiently because most of theamplifier gain is attributed to the lasing
wavelength. The gains that EDFA provided tothe sensing light and to the self-lasing light are
shown in Fig. 4. Blue line is the gain that EDFA
provides in single path optical system. Orange
and green lines are the gains of the sensing
light and the self-lasing light, respectively. As
EDFA pumping current increasing, gain at the
self-lasing wavelength increased from 15 dB to
22 dB, while the gain at the sensing lightwavelength kept at 14 dB.
30

Gain [dB]

Light intensity [a.u.]

PD

Waveguideincreased from 100 mA. As is indicated, thelight intensity at self-lasing wavelength
increased from -53 dBm to -7 dBm whereas theintensity at sensing light wavelength kept at-24 dBm.
Light intensity [dBm]2225

Gain in one-pass optical
Gain at lasing wavelength20

Gain at sensing lightwavelength

15
10
150

200 250300 350 400
EDFA pumping current [mA]450

Fig. 4 EDFA gain at self-lasing wavelength, signal
wavelength, and in single pass optical system.

The “first seed” of the self-lasing is the ASE(amplified spontaneous emission). When the
Er3+ is pumped to produce a population
inversion in EDFA, ASE is generated due to thespontaneous emission. ASE travels inside theamplifier-assisted CRDS. If the travelling ASE
share the same frequency, phase, andpolarization
direction,the“coherency”
condition is secured. Under the coherency
condition, self-lasing happens when the令和 2 年度

九州大学大学院総合理工学報告

travelling ASE intensity reaches lasing
threshold. To suspend the self-lasing, we
propose polarization direction control scheme.
This method is with changing the polarization
direction of the travelling light wave to weaken
the coherency condition. Figure 5 shows theexperimental system of the polarization
direction control scheme. A 3-stage polarization
rotator with a polarizer, a 1/4 wave plate and a
1/2 wave plate is set into the cycle loop after
EDFA. The ASE coming out from EDFA is nonlinear polarization light. The polarizer is used
to alter the ASE polarization state to linear
polarization. The direction of the linear
polarization light is changed by rotating the 1/2
wave plate. The 90° rotation of the polarization
direction mostly weaken the coherency
condition. Hence, self-lasing is suspended.
Several meters

Coupler 1
LD

Polarization
rotator

Waveguide
EDFA

Coupler 2
PD

第 42 巻 第 1 号

23

3.2 Amplifier noise issue and solution
Whereas self-lasing issue was solved, theamplifier noise is a serious problem in theamplifier-assisted CRDS. The amplifier noise
mainly comes from four parts 44): (a) Shot noisecaused by amplified signal light; (b) Shot noisecaused by spontaneous emission light; (c) Beatnoise between signal light and spontaneousemission light; and (d) Beat noise between
spontaneous emission light. Among these fourkinds of amplifier noises, noise (a) and (c) exist
in the same wavelength with the sensing light.
These two kinds of noises are hardly eliminated.
The noise intensity is accumulating while thesenoise looping inside the CRDS. In ppm-ordergas sensing, after more than 1,000 times
looping, the accumulated amplifier noise
intensity may exceed the sensing lightintensity. The intensity of the accumulated
amplifier noise; therefore; directly influences
the sensing ability of the amplifier-assisted
CRDS.
To evaluate the sensing ability of theamplifier-assisted CRDS, firstly, we calculate
the intensity of the amplifier noise. It is
calculated by the amplifier noise figure (NF) 45).
SignalSignal

NF = ( Noise )IN / ( Noise )OUT

Light intensity [dBm]

Fig. 5 Experimental system of polarization direction control
scheme.

(2)

When we change the NF into log-scale in [dB],eq. (3) becomes as follows:

NF = [(Signal)IN –(Signal)OUT] – [(Noise)IN –(Noise)OUT](3)0
Sensinglight@1572n-20-40

The gain G [dB] is expressed by (Signal)OUT and(Signal)IN as:-60

G = (Signal)OUT – (Signal)IN-80(Noise)IN [dBm] is the input thermal noise,
which is expressed as:

-100
1530

1550

1570 1590 1610
Wavelength [nm]1630

Fig. 6 Optical spectrum of polarization direction control
scheme.

The result of the polarization direction
control scheme is confirmed by using optical
spectrum from the output port of coupler 2. The
result is shown in Fig. 6. The self-lasing
intensity was suspended below -50 dBm. We
estimated that the gain at sensing lightwavelength increased to 24 dB from 14 dB.

(Noise)IN = 10×log10(kTB×103)

(4)

(5)

Here, k is Boltzmann constant, T [K] istemperature, and B [Hz] is the bandwidth.
Then the amplifier noise intensity (Noise)OUT is
written as:(Noise)OUT = NF+G+10×log10(kTB×103) (6)In this case, we take NF as 3 dB in thecalculation, because in EDFA the quantum
limit of NF is 3 dB when the population
inversion occurs sufficiently 46). Based on the
co-doped materials of Erbium-doped fiber 45),

Amplifier-assisted waveguide sensing toward breath-gas analysis

EDFA has a stable gain that ranges from 15 dB
to 40 dB. So, we assume that the gain is stable
at 20 dB in this calculation. At the condition of
T = 298 K (25 ℃) and sensing wavelength λ =1572 nm, (Noise)OUT is calculated as -43 dBm(i.e., 50 nW). Then the accumulated noise
intensity In [dBm]is calculated as:

In = 10×log10(n×50×10-6)(7)

where n is the times that the noise looping
inside the system.
To realize ppm-order breath sensing byusing amplifier-assisted CRDS, after n times
looping, the sensing light intensity Is [dBm] isrequired to be larger than the accumulated
noise intensity In. Is is calculated as:

Is = 10×log10{I0×[exp(σNl×Γout)]n }(8)

I0 [mW]is the injection light intensity. l [cm] is
the waveguide length and Γout [%] is the portionof light profiles that comes out of the waveguidefor gas absorption 41). The term of “exp(σNL×Γout)” donates to the gas absorption
attenuation in one loop. The value of Is isavailable to be enhanced by increasing theinjection light intensity I0. We used CO2 as an
example to calculate the necessary injection
light intensity for different CO2 concentration.
The calculated results are shown in Fig. 7. In
this calculation, the system loss and theamplifier gain were all set as 20 dB (ideal
condition). The light intensity criteria was 0.9[a.u.]. The criteria is in inverse proportion to
the cavity ring-down number. A higher criteria(~1 [a.u.]) leads to less cavity ring-down
number. By using the criteria as 0.9 in thisestimation, the accumulated amplifier noise
intensity is low. The blue line is theaccumulated amplifier noise. The blue dot lineis the “noise intensity+ 3dB”. It is the lightintensity level to distinguish the sensing lightand the noise.
Light intensity [mW]

2

10

1

10

0

10

-1
-2

10

400ppm CO2 I0=0.6mW
ring-down number 5,143

-3

10

0

4

4

2

10

5ppm CH3COCH3 I0=9.8mW

1

10

0

Noise+3dB

10

-1

10

5ppm CH4 I0=0.55mW
5ppm NH3 I0=0.14mW

-2

10

-3

10

0

Accumulated noise

4% CO2 I0=0.006mW
4

4

4

4

2×10
4×10 6×10 8×10
Cavity ring-down number

5

1×10

Fig. 8 The calculated results of the necessary injection lightintensity for breath contents (CO2, CH4, NH3, andCH3COCH3).

In this figure, the orange, green, red, andyellow line show the sensing light intensity
decreasing tendency for CH3COCH3, CH4, NH3,
and CO2, respectively. Based on the estimation
above, the concentration of CO2 in humanbreath (40,000 ppm) is detectable with 0.006
mW injection light intensity. The CH3COCH3,
CH4, NH3 in exhaled breath are detectable with
injection light below 10 mW.
Most of the EDFA provides a stable gain
when input light intensity is in between -40
dBm~-20 dBm 44,45,47). The injection lightintensity I0 has an intensity limitation toensure the stable gain of amplifier. The lightintensity range of I0 is estimated as: 0 dBm (1mW) ≤ I0 ≤ 10 dBm (10 mW), considering theloss of the waveguide (the loss is estimated as-20 dB). According to the analysis above, ppmorder gas concentration is detectable with the
influence of amplifier noise. Amplifier-assisted
CRDS is available for ppm-order gas sensing,
as long as we control the injection lightintensity.

4. CO2 gas concentration
measurement

40ppm CO2 I0=6mW
ring-down number 51,428
Noise+3dB
Accumulated noise

10

When the injection light intensity increased
from 0.6 mW to 6 mW, the detectable CO2
concentration changed from 400 ppm to 40 ppm.
The necessary injection light intensity I0 for
breath contents (CO2, CH4, NH3, andCH3COCH3) are shown in Fig. 8.
Light intensity [mW]

24

4

2×10
4×10
6×10
Cavity ring-down number
Fig. 7 The calculated results of the necessary injection lightintensity for different CO2 concentration.

Based on the calculation results above, one
requirement is to use sufficient light intensity.
For instance, I0 is required to be more than
0.006 mW for CO2. In this case, CO2concentration in human breath (40,000 ppm) isavailable for sensing. To confirm the actual
availability, we tried 3% CO2 gas sensing byusing the amplifier-assisted CRDS with 1 mW

令和 2 年度

九州大学大学院総合理工学報告第 42 巻 第 1 号

25

injection light-pulse. The light-pulse has awidth of 10 ns and a period of 20 μs.

proposed amplifier-assisted
breath sensing.

The sensing waveguide was silica highmesa waveguide integrated on a Si wafer.
Figure 9 (a) is structure of the integrated
waveguide chip. The chip size is 0.9 cm × 0.9 cm.
Figure 9 (b) is the structure of silica high-mesa
waveguide. The length and width of the
waveguide that was used in sensing
experiment was 36.8 cm and 2.23 μm,
respectively.
Theevaluatedwaveguide
propagation loss was 0.02 dB/cm. Figure 9 (c)shows the waveguide simulated optical field.
The light intensity out of waveguide ( Γout) is
estimated as 3% of the total light intensity.

Theestimated
CO2
concentration
corresponds to be 1%, and the value was notexactly same with the injected gas condition.
This difference may be because of the error in
the accuracy of CO2 density. The 3% CO2 that
injected into the gas chamber was not proper
3% so that the sensing error happened. We are
now on the way of the analysis of this difference.

0.9 cm(a)

Waveguide total length 36.8 cm
2.23 μm2.23 μm

SiO2
Ge-doped
SiO2
SiO2
Si
(b)

enableCavity ring-down number

1.0Light intensity [a.u.]

0.9 cm

CRDS

0

60

120

180

240

300

W/O CO2With CO2
W/O CO2 approximate line
With CO2 approximate line

0.8
0.6
0.4
0.2
0

0

20

40
60
Time [μs]80100Fig. 10 3% CO2 analyzed sensing results.

5. Conclusions(c)Fig. 9 High-mesa waveguide structure. (a) High-mesa
waveguide chip, (b) waveguide structure, and (c) waveguidesimulated optical field.

In order to analyze the light intensity of thepulse-train, we took the light intensity data ofeach pulse peak. By using these data, we draw
the approximate line. The analyzed results areshown in Fig. 10. The blue dots and green
triangles in Fig. 10 are the light intensity dataof “no CO2” and “3% CO2” situation,
respectively. The orange line and the yellow
line are the approximate lines. The cavity ringdown times for gas concentration measurement
is defined by the light intensity criteria that weset. Here, the criteria level is set at the lightintensity ~1/e (red line in Fig. 10). This isbecause this value is near to the normal criteria
level 1/e and easy for cavity ring-down timemeasurement. The cavity ring-down times were
estimated as 80.7 μs and 41 μs for “no CO2” and“3% CO2" situation, respectively. Because
human breath contains 4% CO2, in this work,
the result of 3% CO2 sensing indicates that the

Amplifier-assisted
waveguideschemeutilizing CRDS for gas sensing realizes ppmorder gas sensing. Amplifier inside CRDS;
however; causes self-lasing issue and amplifier
noise issue. These two issues prevent ppmorder gas sensing. We have proposed
polarization direction control to suspend selflasing. The results showed that self-lasing wassuspended below -50 dBm. The intensity ofaccumulated amplifier noise directly influences
the sensing ability of the amplifier-assisted
waveguide scheme. The amplifier noise ishardly eliminated because it exists at sensing
light wavelength. One requirement of gas
sensing under the influence of amplifier noise
is to use sufficient light intensity. Based on thecalculation, injection light intensity within 10
mW is available for several kind of gas sensing(CO2, CH4, NH3, and CH3COCH3). We
successfully detected 3% CO2 using 1 mW input
sensing light intensity as a ring-down timedifference (80.7 µs and 41 µs) based on the
proposed amplifier-assisted CRDS. This result
indicates that the amplifier-assisted CRDS isavailable for breath sensing.

26

Amplifier-assisted waveguide sensing toward breath-gas analysis

6. Acknowledgments
This work has been supported in finance by
SECOM Science and Technology Foundation.
The authors greatly appreciate Drs. Mikitaka
Itoh and Yasuaki Hashizume of NTT for theirtechnical discussion and support.

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