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SSSeeennnsssooorrrsss &&& TTTrrraaannnsssddduuuccceeerrrsss

© 2014 by IFSA http://www.sensorsportal.com

Research of the PPM Modulation Technology in Space Communication

* Yan WANG, Hongzuo LI and Ziqiang HAO

Changchun University of Science and Technology, Changchun, China Tel.: 13086801212, fax: 13086801212

E-mail: Juvener1985@163.com

Received: 19 December 2013 /Accepted: 28 January 2014 /Published: 28 February 2014 Abstract: Fiber laser has the characteristics of high peak power, short Pulse-width and good beam quality etc., and play a larger role in the field of laser communication, laser cutting, medical instruments etc. This paper first study on theoretical research of pulse single pulse energy, peak power, pulse width of the Q-switched fiber laser, and characteristic curve between the pulse peak power of Q-switched fiber laser and the laser shock parameters were obtained through simulation, combined with L-PPM signal model analysis the best selection of L-PPM digits in the repetition frequency of the laser under certain conditions, through the research on some important parameters such as slot width of PPM signal, length of transmission slots, length of guard slots etc., arrival at a conclusion of the modulation characteristics of PPM based on fiber laser, through theoretical analysis and experimental simulation, proved that under the condition of the lower repetition frequency PPM can provide a higher modulation rate. Obtain that when the pulse width of the fiber laser is 3 ns, the repetition frequency is 200 kHz the modulation rate of the PPM system can reach the value of 1.3 Mbit/s. From above we can know that use the fiber laser with PPM modulation method can effectively improve the modulation rate of the system. Copyright © 2014 IFSA Publishing, S. L. Keywords: Optical communications, PPM modulation, Fiber laser, Q-switched., Space communication. 1. Introduction

In order to meet the requirements for long-range data transmission of space optical communication, it is necessary to maximize the transmission power and conversion efficiency of emitted light [1]. The characteristics of fiber lasers are very valuable resources for space optical communication, such as high peak power, good beam quality and short pulse-width etc. But because of its relatively low repetition frequency, it is difficult to achieve the requirements of high-rate communication [2, 3]. At a given average power of the light, PPM (Pulse Position Modulation) can achieve a high data transfer rate with a small laser pulse repetition frequency, which

makes up for the shortcomings of the fiber laser. It also has the advantages of high power utilization, high transmission efficiency and anti-jamming ability etc. which makes it widely used in the field of laser communication. So PPM technology based on fiber lasers will become a focus in space optical communication.

In this paper, pulse Q-switched fiber laser is used. Starting with the nature of the laser itself, combined with L-PPM signal expression, PPM characteristics of fiber lasers are analyzed.

The results shows that in the fiber laser transmission system, transfer rate can reach 1.3 Mbit/s by using PPM, which is the transfer rate can be improved.

Article number P_1850

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2. Q-switched Fiber Lasers

The emergence and application of Q-switching is a major breakthrough in laser history. Since the 1961 Hellwarth and McClung of Columbia University rose up the concept of Q-switching, the technical theory of Q-switched fiber laser is already quite mature after the development of about half a century. Because of their characteristic of narrow pulse-width, high pulse energy and high peak power, they are broadly used in laser communications, optical precision ranging, laser radar and high-speed photography and other fields [4]. Also, because they can produce MW-level peak power and W-level average power, Q-switched fiber lasers will become a research focus in space laser communications.

General pulse width and peak power of Q-switched lasers output is respectively in nanoseconds and MW magnitude [5]. In this paper the structure of acousto-optic Q-switched fiber laser is shown in Fig. 1, in which the working substance is Cr4+:YAG crystal.

Fig. 1. The conformation of A-O Q-switched laser.

As the modulation used in this paper is PPM, so when analyzing fiber laser, we have to analyze the related characteristics of a single pulse signal from fiber laser. So this section focuses on the analysis on energy Ep and peak power Pmax of the fiber laser pulse.

First of all, according to the literature [6], coupled output instantaneous power is P(t):

'

'

1ln( )

( ) ( )

r

dhvAl

d dt RP t hvAl t

dt tR

(1)

2 '

0

2'

0

ln(1 / ) ln(1 / )ln( )

2( )

(1 ) ln(1 / ) 11 ln( )

2

i

i

i

T R L nn n

l nln

T nl

l n

(2)

Therefore, the expression of single pulse energy Ep is as follows:

0 0

1 1ln( ) ln( )

( ) ( )

1ln( ) ln( )

2

ni

p n f

r r

i

f

hvAl hvAldnR R

E P t dt t dtt ct n

hvA n

R n

(3)

By the basic formula of acousto-optic Q-switched

lasers [6-8], the following equation can be introduced:

max max

1ln( )

1 1ln( ) {1 ln( ) (1 )[1 ( ) ]}

r

t t t

i

r i i i

hvAlR

Pt

hvAl n n nn N N

t R n n n

(4)

'

2r

lt

c

(5)

Let n be the density of initial inversion in laser

material when the Q-switched is turned on. Let ni be the density of initial inversion in laser material when Q-switched is passively turned on. The density of the threshold inversion becomes a lower value nt when Q switch is turned on from the larger one before Q switch is turned on. nt is the density of threshold inversion during the forming process of Q-switched pulse, nf is the density of inversion when Q pulse disappears.

2 '

0ln(1 / ) ln(1 / )

2i

T R Ln

l

(6)

The density of threshold inversion t

n in

forming process of Q pulse is denoted as nt0

when .

2 '

0

0

ln(1 / ) ln(1 / )

2t t

T R Ln n

l

(7)

(1 )( )t t

i i

n nN N

n n

(8)

1

1 ( ) (1 )[1 ( ) ]f f f

i i i

n n nN N

n n n

(9)

Since the working substance used in Q-switched

lasers is Cr4+:YAG in this paper, by settings relevant physical parameters of Cr4+:YAG crystal (such as ground state of saturated absorption medium/excited state absorption cross-section, the loss factor, the laser medium stimulated emission cross sections, etc., as shown in Table 1), and assuming that conversion efficiency of laser light-optical in the process of

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pumping is 20 %, peak jitter of the amplitude caused by the pumping spontaneous emission is 4 %, frequency stability is 1 %. In this condition, the relationship between peak power Pmax and N can be obtained by the numerical simulation for equation (4) (the relevant parameters is in Table 1) as illustrated in Fig. 2. Table 1. Spectral parameters of Q-switched fiber laser used

in numerical simulations.

Parameters Meaning Values

gs

Groundstate cross section of saturated absorbing medium

8.7×10-18 (cm2)

es

Excited state cross section of saturated absorbing medium

2.2×10-18 (cm2)

Hv Laser photon energy 1.86×10-19 (J)

A Effective beam cross-sectional area

π(0.2 cm)2

L laser rod length 0.04 (cm) R Output mirror reflectivity 20 %

Σ Stimulated emission cross section

2.8×10-19 (cm2)

l’ Resonator length 170 (mm) Γ Inversion reduction factor 1

T0 Initial transmissivity of laser crystal

30 %

L’ Dissipative optical loss 0.02 (cm-1) C Speed of light 3×108 (m/s)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

N

Pm

ax

α→∞

α=2.5α=1.5

Fig. 2. The peak power Pmax and N with different values of α.

Pulse width τ can be obtained approximately by dividing the peak power of the pulse energy, namely τ=Ep / Pmax. According to Equation (3) and Equation (4), there are:

max

'

2

0

11 (1 )[1 ( ) ]

.{ }1 1 1

ln( ) ln( ) 1 ln( ) (1 )[1 ( ) ]

p

f f

r i i

t t t

i i i

E

P

n nN

t n n

n n nL N N

T R n n n

(10)

As the pulse width of fiber laser used here is τ=3 ns, maximum pulse width is τm=10 ns, relationship between the pulse width τ and N is obtained when the pulse width ranges from 0 to 10 ns as illustrated in Fig. 3 by numerical simulation according to the formula (10) (relevant parameters as shown in Table 1):

0 0.1 0.2 0.3 0.4 0.5 0.6 0.70

1

2

3

4

5

6

7

8

9

10

N（

）τ

ns

α→∞

α=4

α=1.5

Fig. 3. The pulse width and N with different values of α.

Let α be the specific parameters of passively Q-switched lasers, it means that if it is easy for the bleaching of saturable absorber. When α is larger, the saturable absorber can be more easily bleached (easier to achieve a good functioning of Q), the expression is:

gs

(11)

N can be seen as the shock parameters of

Q-switched fiber laser, namely: N is the ratio of threshold inversion density after the Q switch is turned on and initial inversion density before the Q switch is turned on when , which is determined by the saturable absorption body and resonator parameters. Its expression is:

'

2

0 0

'

2

0

1 1ln( ) ln( )

1 1ln( ) ln( )

t

i

Ln T R

Nn

LT R

(12)

where

es

gs

(13)

Fig. 2 and Fig. 3 show that peak power and pulse

width of the Q pulse are directly or indirectly

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dependent on two parameters: N and α. It also prepares the way for the following analysis on signal characteristics of PPM. 3. Analysis of PPM

Usually, we call the process of loading information to be transmitted to laser radiation as laser modulation, since the laser act as the role of a ‘information carrier’, so it is called carrier. For the message to be transmitted usually is called modulation signal. The modulated laser is called modulated wave or modulated light. Modulation characteristics are defined as the relationship of the laser output signal changing with the modulation signal. This paper uses pulsed Q-switched laser, by adjusting the different pulse repetition frequency, the laser output signal is observed to make the laser steady working in the most efficient modulating point.

PPM uses periodic optical pulses as the carrier, the carrier transmits information by the change of the pulse position under the control of modulated signal. Compared to OOK, it has higher energy transfer rate, good anti-interference ability, it greatly reduces requirements for the average power of transmission laser, it can achieve better average power utilization, and the encoding circuit is simple and easy to implement. In laser communications, this form of modulation can reach a high data transfer rate with a small average light power at a given pulse repetition frequency of the laser. The study shows that the channel error immunity capacity of PPM significantly enhances besides improving the efficiency of laser power. It is noted that MPPM requires smaller bandwidth than L-PPM and DPPM, but implementing MPPM and DPPM is much more complex than L-PPM, and the demodulation is more difficult. In summary, this paper selects L-PPM as the modulation of space laser communication [9]. 3.1. Analysis of L-PPM Signal

According to the definition of L-PPM, L-PPM modulation signal can be expressed as [10]:

( 1)( )

0c c

n

P L t LcS tother time

(14)

Where Sn(t) represents the PPM modulation

signal, Pc is a single pulse power, τc is time slot width.

When the laser emits light pulse in actual laser communication system, there is a small time interval between two adjacent pulses, called a guard slot (TD). Because the laser requires a minimum delay time for rebuilding a ‘reverse beam’. If the n bits information are coded into L=2n time slots through L-PPM, one frame period of L-PPM consists of

L transmission slots and D guard slots, the frame structure is shown in Fig. 4:

信息时段 保护时段

DTD LTS

光脉冲

MTf

Fig. 4. The conformation of PPM frame.

Let the hexadecimal number of L-PPM be L, the n-bit data to be sent is M=(m1, m2,…mn), while one L-PPM frame period is divided into transmission slots and guard slots, and transmission slot is of a total of L slots, the width of each slot is denoted as τc, the guard slot is denoted as TD, then frame period of the system can be [11].

PPM c D

T L T

(15)

The frame rate of the system is:

1

R c Df L T

(16)

The messaging rate of system is:

2

logc c D

R L L T (17)

According to the formula (17):

2 2 2 2log log log log

2 ( ) ( )c n

s D c D c

L L L LR

T T T L D L D

(18)

where τ is the pulse width of the laser, let μ be the pulse duty ratio, τ=μτc. Since the purpose of this paper is to specify the use of fiber-based laser PPM modulation scheme can effectively improve the transmission rate, then the analysis of transmission rate Rc is particularly necessary. We can see from the equation (18), the transfer rate Rc are mainly determined by L in L-PPM modulation, protection section D, the pulse width τ and L-PPM slot width τc. Therefore, the analyzed signal parameters of L-PPM are L-PPM modulation median L, relationships of transmission slots L and guard slots D, and pulse duty cycle μ. 3.2. L-PPM Parameter Analysis

As modulating the laser, the optical pulse width is generally not equal to the width of a slot, assuming the light pulse width from a pulsed fiber laser is τ, then τ ≤τc. Fig. 5 is the simulation diagram according to the formula Rc=μlog2L/(Lτ+1/f), where let L=256,f=200 kHz, τ=3 ns, τ =μτc. As seen from the

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186

figure, the system transfer rate Rc increases with μ increasing.

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

μ

（）

Rc

Mbi

t/s

Fig. 5 The diagram between μ and Rc.

Based on the above analysis and consideration, the parameter μ between pulse width of the fiber laser and the slot width of PPM modulation system can be taken less than and as close as possible to a value of 1. That is to say slot width τc should be slightly larger than the pulse width τ, τc-τ should be close to 0.

According to the definition of repetition frequency, the reciprocal of the shortest time interval from a pulse to the next pulse is called the pulse repetition frequency. The minimum time interval mentioned in the definition can be understood as when a single pulse in a sequence of L-PPM just is the last one of 2n slots, and the next single pulse is just the first one of 2n slots, the time between the two single pulses is the minimum, the shortest time is guard slot TD, then the repetition frequency is f=1/TD [12].

Where n=log2L is the modulation bits of L-PPM modulation system. After derivation for formula (18) it can be obtained:

2 ( ln 2 1)n

c Dn T

(19)

According to the formula (19), the relationship

between modulation bits of the system and repetition frequency of fiber laser is illustrated in Fig. 6.

Where f=1/ TD, shows that modulation bits of L-PPM system is relevant with laser repetition frequency f and slot width τc. We set the repetition rate of fiber laser as 200 kHz, pulse width τ=3 ns. At first f=1/ TD =200 kHz, τc=τ=3 ns, it can be seen in Fig. 6 when f=1/ TD =200 kHz, n≈8.4.The equation (19) shows that when TD is unchanged τc is inversely proportional to n. As τ ≤τc, τc can only bigger than τ (3 ns), so that n is an integer only less than 8.4 τc. According to the value τ should ensure τc–τ tends – to 0 which has analyzed above, that is to say τc–τ should be as small as possible, so n should take an integer less than 8.4 and the nearest from 8.4. according to the parameters of fiber laser this paper

provided, it should be taken n=8. So this paper selects L-PPM modulation median of eight, namely 256-PPM.

4 5 6 7 8 9 100

100

200

300

400

500

600

700

modulation digit n

repe

titio

n fr

eque

ncy(

kHz)

Fig. 6. The diagram between modulation digit and repetition frequency.

Since this paper selects 8 L-PPM modulation system, so it can be obtained τc≈4.25 ns taking n=8 into Equation (19). Because of f=1/ TD =200 kHz, while TD =D, so that the length D of guard slots is about 1176, while the length of transmission slots is L=2n=256.So when the modulation bits n of L-PPM modulation system and the laser repetition frequency f is unchanged, the best ratio between transmission slot length L of L-PPM modulation system and the slot length of guard slot D is: L:D=256:1176≈1:4.6. Similarly can be drawn, the ratio μ of the pulse width τ and slot width τc should be taken μ=3/4.25=12/17. 3.3. PPM Modulation Characteristics

of Fiber Lasers

The so-called modulation characteristics are the relationship of laser output laser signal and the oscillation parameters with the changes of modulated signal [15]. Here we analyze the PPM modulation characteristics of fiber lasers from the variation of the laser shock frequency N and laser repetition parameters f with L-PPM signaling transmission rate.

According to the formula (10) and formula (18), there is:

'

2

2 0

1 1ln( ) ln( )

log

( )

11 ln( ) (1 )[1 ( ) ]

{ }1

1 (1 )[1 ( ) ]

c

r

t t t

i i i

f f

i i

LL T R

RL D t

n n nN N

n n n

n nN

n n

(20)

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By the formula (20) the relationship between transfer rate Rc and N can be obtained (relevant parameters are in Table 1), as shown in Fig. 7:

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

N

（）

Rc

Mbi

t/s

α→∞

α=4

α=1.5

Fig. 7. The modulation rate Rc and N with different values of α.

Fig. 7 shows the relationship between transmission rate of 256-PPM modulation system Rc and N when L=256, D=1176, μ=12/17, L:D=256:1176≈1:4.6, and α were respectively taken 1.5, 4 and ∞. As seen in Fig. 6 and Fig. 7, the transmission rate of L-PPM can be unlimited big when the laser pulse width is infinitely small. In the actual situation, the laser pulse width cannot be infinitely small. But it still gives us a theoretical basis for improving the transmission rate, laser pulse output should be as narrow possible, which is one of the reasons why we chose the fiber laser as PPM emission source. As seen in Fig. 7, whatever α is, there are some N making transfer rate of L-PPM up to more than 1.3 Mbit/s, which indicates that the pulse fiber laser can effectively improve the transmission rate of L-PPM system.

Here we continue to analyze the changes relationship of repetition frequency f of fiber laser and signal transition rate Rc of L-PPM.

By equation (17) we know Rc=n/T= n/ (L+D)τc, where n is the modulation bits, L=2n. And by equation (18) we know τc =TD /2n(nln2-1), where TD =D, τc =1/ f. f is the repetition frequency of the laser. So we can get:

( ) 2 / 2 ( ln 2 1) 1 /

2 ( ln 2 1)

2 ln 2

c n n

c D

n

n

n nR

L D T n f

n nf

n

(21)

Fig. 8 shows the relationship between PPM modulation rate and repetition frequency when the modulation bits is n=8. As seen from the figure, when the modulation bits are unchanged, L-PPM transmission rate increases with the increase of laser

repetition frequency. When the laser repetition frequency is 1000 kHz, the system transmission rate can reach 6.5 Mbit/s. So PPM can provide a higher transfer rate in the condition that the laser output is a low repetition frequency. This is the main reason the paper selects PPM. Also it can be seen from the figure the laser repetition frequency is 200 kHz, when, the transmission rate of L-PPM modulation system can reach 1.3 Mbit/s. In a real system, it is easily achieved to make the repetition frequency of fiber laser up to 200 kHz, so from the view of the relationship between repetition frequency of fiber laser and the transition rate of L-PPM signal can also prove pulsed fiber lasers can effectively improve the transmission rate of PPM system.

101

102

103

0

1000

2000

3000

4000

5000

6000

7000

Repetition frequency (kHz)

Rc

(kbi

t/s)

Fig. 8. The relationship between PPM modulation rate and repetition frequency.

4. Experiment Simulation 4.1. System Simulation

The simulation is performed by OptiSystem, which is optical simulation software. We set the wavelength of seed light source as 1064 nm, from the experimental system we know the peak power is 1000 W, the pulse width is 3 ns, the laser output power is 602 mW. The purpose of this simulation is to observe the value change of system repetition frequency f and the transmission rate Rc by setting different system repetition frequency, in order to prove the correctness of above characteristics of PPM.

The repetition frequency of fiber laser is 200 kHz and the transmission rate of L-PPM modulation system is about 1.325 Mbit/s, the fiber laser can effectively improve the transmission rate of PPM system; when the transmission rate of L-PPM system up to 6.473 Mbit/s, it is necessary to provide repetition frequency of about 1000 kHz for fiber laser, it indicates PPM can provide a higher transfer rate under conditions of the laser output low

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frequency. This simulation results is consistent with the mentioned modulation PPM characteristics of the fiber laser, which indicates that the use of PPM technique based on fiber laser allows the system to achieve a higher transfer rate.

4.2. System Experiment

The designed fiber laser PPM modulation system is to illustrate the transmission rate of PPM system can up to 1.3 Mbit/s using fiber lasers. In this paper, direct modulation method is adopted, also FPGA is used. The experimental apparatus includes: a computer, one FPGA logic control panel, a pulsed Q-switched fiber laser, a photoelectric detector, an oscilloscope, a power meter, and its connection is

shown in Fig. 9. The oscilloscope used is Tektronix DPO7104, it has three bandwidth models of 1 GHz. It can achieve 10 GS/s real-time sampling rate at four channels to be able to observe transient phenomena. The waveform of sent data, PWM waveform and PPM waveforms can be observed in this oscilloscope.

Fig. 9 is a test program, the computer transmissions binary data to FPFA system board through the serial port. FPGA converts the input pulse signal to PPM signal, and then a PPM pulse signal adapted to fiber laser is generated through the trigger plastic, then the PPM signal is modulated to the fiber laser directly, and then we detect the output laser pulse by the photoelectric detector, at last we view the waveform of modulated signal using the oscilloscope.

Fig. 9. Diagram of the system with fiber laser and PPM modulation.

The oscilloscope can display the pulse width of PPM signal and repetition frequency of Q-switched fiber laser, and the power meter can measure the pump power and the value of the laser output power. According to the experimental values and formula (17), we can calculate the value of the transfer rate Rc, which can judge if the PPM based on fiber lasers can improve the transmission rate. Table 2 shows the relationship of input power, output power and the pulse width of the Q-switched fiber laser when the pump current is 2.5 A.

As seen from Table 2, the input power is inversely proportional to output power and pulse width. Since the pulse width of laser is 3 ns, which can be seen from Table 2, the pump power required in this paper is 3.5 W, while the output average power of the laser is 602 mW.

Table 2. The output power and pulse width with different

input power.

Ppump (W) Pout (mW) τ (ns) 2.5 484 3.95 3.0 565 3.41 3.5 602 3.08

As shown in Fig. 10, the yellow part of the first

line represents the pulse signal of PPM, which is a yellow pulse represents a PPM pulse; In order to facilitate the observation and description, we design a line of analog frame signal (second blue row).

Analog frame signal in this paper does not have real meaning, it only played the role to facilitate the observation and description. It can be seen the

distance between every two adjacent analog frame signals is equal, it represents frame length of L-PPM signal. As seen from the figure, the position of each yellow PPM pulse is at the corresponding frame of the two blue analog signal pulses, and you can see the pulse positions between the frame of blue analog signal where each of the PPM yellow signal are not the same. Therefore, there is only one signal on one slot position at a fixed frame period, and pulse positions are different. The pulse positions are corresponded with sent information, PPM is the method to modulate signal through pulse positions. As seen in Fig. 10, the pulses gap time between every two blue analog frame signal in second lines can be considered as period of PPM pulse signal, it is 5 μs, so the repetition frequency of fiber laser is about 200 kHz.

Fig. 10. The waveform of PPM modulation.

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As shown in Fig. 11, it displays waveform of a single pulse PPM signal. As seen from the figure, pulse width of PPM signal is 3 ns. Besides, the repetition period of L-PPM signal is about 5 μs which can be seen from Fig. 11. The average power of laser is 602 mW which can be seen from Table 2. Therefore, the peak power required by the system is 1000 W by the equations Pout=Pmaxτ/ T and Rc=n/ T, the data transmission rate can reach 1.3 Mbit/s.

Fig. 11. The waveform for single pulse of PPM.

5. Conclusion

In the launch systems of space optical communication, the key of the whole system is to select the appropriate laser light source and modulation mode. In this paper, L-PPM modulation transmission system with pulsed Q-switched fiber lasers is adopted, and we find the changes relationship between the laser parameters (e.g., the oscillation parameters N and repetition frequency f) and L-PPM parameters (such as transmission rate Rc), starting from the input L-PPM signal to the fiber laser, through the analysis on own characteristics of pulsed Q-switched fiber laser and L-PPM related parameters. Theoretical analysis and experimental simulation show that transmission rate of PPM system can reach 1.3 Mbit/s. when the pulse width of fiber laser is 3 ns and repetition frequency is 200 kHz. It illustrates the approach of combining fiber lasers and PPM can effectively improve the transfer rate.

Acknowledgment

The authors wish to thank the helpful comments and suggestions from my teachers and colleagues in Changchun University of Science and Technology at Changchun. And also thank The Space Technology lab in Changchun Up-tech to provide experimental environment.

References [1]. Jiang Huilin, Tong Shoufeng, The Technologies and

system of space laser communication, National Defense Industry Press, Beijing, 2010, pp. 9-21.

[2]. J. B. Liang, X. D. Gao, D. Y. You, et al., Detection of seam offset based on Molten pool characteristics during high-power fiber laser welding, Advanced Materials Research, Vol. 549, 2012, pp. 1064-1068.

[3]. Lou Qihong, Zhou Jun, Zhang Haibo, Yuan Zhijun, Recent progress of large core fiber lasers, Chinese Journal of Lasers, Vol. 37, No. 9, 2010, pp. 2235-2241.

[4]. Yang Xiaodong, Hou Xinhua, Investigation on the output characteristics of LD end-pumped Cr4+: YAG passively Q-switched laser, Acta Pfocusonica Sinica, Vol. 41, No. 10, 2012, pp. 1145-1148.

[5]. Ning Jiping, Zhang Weiyi, Shang Lianju, Fan Guofang, Han Qun, Zhou Lei, All-fiber Q-switched ytterbium-doped double-clad laser, Chinese Journal of Lasers, Vol. 35, No. 4, 2008, pp. 483-487.

[6]. J. J. Degnan, Theory of the optimally coupled Q-switched laser, IEEE Journal of Quantum Electronics, Vol. 25, Issue 2, 1989, pp. 214-220.

[7]. Xu Tao, Jin Guangyong, Yu Yongji, Chen Xinyu, Wang Chao, Wu Chunting, Study on the pulse interval of passively Q-switched laser pumped by diode laser pulse, Laser & Optoelectronics Progress, Vol. 49, Issue 6, 2012, article ID: 061401.

[8]. Wang Zhimin, Xu Jiangiu, Chen Weibiao, High-power passively Q-switch ultra-thin slab lasers, Chinese Optics Letters, Vol. 5, Issue S1, 2007, pp. S36-S38.

[9]. X. Liu, T. H. Wood, R. W. Tkach, et al., Demonstration of record sensitivities in optically preamplified receivers by combining PDM-QPSK and M-ary pulse-position modulation, Journal of Lightwave Technology, Vol. 30, Issue 4, 2012, pp. 406-413.

[10]. M. L. Stevens, D. M. Boroson, A simple delay-line 4-PPM demodulator with near-optimum performance, Optics Express, Vol. 20, Issue 5, 2012, pp. 5270-5280.

[11]. M. Rouissat, R. A. Borsali, Differential two-pulses position modulation for synchronized wireless optical communications, Computer Networks, Vol. 370, 2013, pp. 252-257.

[12]. Zhang Huawei, Jia Honghui, The analysis of L-PPM modulation for ultraviolet laser communication, Optical Communication Technology, Vol. 34, Issue 5, 2010, pp. 56-58.

[13]. S. J. Savage, B. S. Robinson, D. O. Caplan, et al., Scalable modulator for frequency shift keying in free space optical communications, Optics Express, Vol. 21, Issue 3, 2013, pp. 3342-3353.

[14]. Wu Jili, Zhao Shanghong, Xu Jie, Li Yongjun, Study of capacity of coherent pulse-position modulation channel and maximization of information transmitionting rate, Acta Optica Sinica, Vol. 28, No. 4, 2008, pp. 643-647.

[15]. Fan Yangyu, Bai Bo, Huang Aiping, Tian Hua, Li Long, Li Xiaojun, Pulse-position-width modulation scheme in wireless optical communication system, Chinese Journal of Lasers, Vol. 35, Issue 12, 2008, pp. 1883-1887.

Sensors & Transducers, Vol. 164, Issue 2, February 2014, pp. 182-190

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