Development of a 488-nm Blue Laser for Biomedical Analyzers
by Takeshi Takagi *, Hiroshi Matsuura *, Akira Fujisaki * and Tomoya Kato *2

The authors have developed a 488-nm blue laser by combining a 976-nm semiconductor laser Furukawa Electric has long fostered in the optical communications field with a wavelength conversion device based on periodically poled lithium niobate (PPLN). Because of its simple device configuration whereby the semiconductor laser
for optical communications —where long-term reliability up to 20 years is required— is directly
coupled with the wavelength conversion device, the laser has a long service life, compact size,
low cost, and low power consumption. Moreover, by optimizing the spectrum of fundamental
lightwave and by providing a feedback circuit in which a fraction of the output light is monitored
for control use, the laser has achieved both high output stability and low noise that were unattainable for conventional argon lasers and diode pumped solid state (DPSS) lasers. The
obtained performance, i.e. output power ≥20 mW, output power stability <2 %, peak-to-peaknoise < 1 %, and rms noise < 0.25 %, was confirmed to meet the characteristics required forlight sources for biomedical analyzers. In the future strong demand is expected as a light sourcefor bioinstrumentation.

ABSTRACT1.

INTRODUCTION

Recently, with the aging of society and the advancementof medical technologies such as gene-analysis, the bioanalysis equipment industry continues to expand. And
flow cytometer (FCM) is one of these equipment for cellanalysis.
FCM illuminates cells with a laser beam to analyze andsort the cells based on the scattered light and fluorescence, using a 488-nm blue laser as the light source for
exciting the fluorescence. Although argon laser constituted the mainstream of 488-nm laser light sources throughthe ages, it had problems specific to gas lasers such as
short service life, large size, noise, vibration, high power
consumption, heat generation, and need for regular maintenance, so that there has been a strong need for a solidstate blue laser for bioinstrumentation.
DPSS laser has been developed and marketed in recent
years, but the laser has not solved the issue of heat generation nor the difficulty in cost reduction due to its structure.
Furukawa Electric has an impressive track record in thefield of optical telecommunications, in the manufacture of
980-nm pumping lasers for optical communications andoptical modules, and taking advantage of these technologies, the Company has lately developed a 488-nm blue
laser to replace gas lasers. The new laser uses a 976-nm
semiconductor laser shown in Figure 1 for fundamental* FITEL-Photonics Lab., R&D Div.
* 2 Corporate Strategy Planning Dept.

Figure 1976-nm semiconductor laser.

lightwave generation and a PPLN device for second harmonic generation (SHG), thereby outperforming bothargon laser and DPSS laser. The developmental work ofthis blue laser light source will be described in this paper.

2.

CONFIGURATION OF 488-NM BLUELASER

Figure 2 shows the configuration of the blue laser developed here. A 976-nm semiconductor laser is employed as
fundamental lightwave generator, and after wavelengthstabilization using fiber Bragg grating (FBG), the source
light is directly coupled to a SHG wavelength conversion
device of planar waveguide type.
Because, in single-longitudinal-mode operation, the976-nm semiconductor laser is subject to mode hopping
due to fluctuations in the driving current, the laser is provided with an anti-mode hopping structure of multi-longitudinal-mode, and moreover, the reflection characteristics

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27

Development of a 488-nm Blue Laser for Biomedical Analyzers

PMF

PMF
976 nm LD

OutputPPLN
PD

ILD: control

TLD: const. TPPLN: const.

Feedback

Control circuit

Figure 2

Configuration of 488-nm blue laser.

of the FBG are optimized to stabilize the wavelength. In
this way second harmonic generation of high efficiency
and stability has been achieved despite the use of PPLN
which is known for its sensitivity to wavelength changes.
Moreover, since PPLN is also sensitive to temperaturechanges, the device temperature is controlled within arange of 0.5°C using a Peltier cooler.
Each device is optically connected using a polarization
maintaining fiber (PMF) for specific use. In particular, theasymmetrical beam output from the PPLN waveguide isideally shaped into a symmetrical beam by the use ofPMF.
In addition, to stabilize the optical output power, theblue light output from the PPLN waveguide, monitored bya photo diode, is used to control the output power suppressing its fluctuation.

3.

DEVELOPMENT TARGETS

Table 1 shows the target specifications of this development. The specifications were established, making reference to those of argon lasers and DPSS lasers used in
the existing biomedical applications, to satisfy the requirements for bio-analysis light sources.

become important. Theoretically, high conversion efficiency could be obtained by narrowing the line width of fundamental lightwave thereby increasing the peak power,
but this would result in unstable output power due to fluctuations in the spectrum of fundamental lightwave.
Because the allowable range of wavelength conversion
device is rather narrow, fluctuations in the spectrum offundamental lightwave significantly influence the SHG
output power. Moreover, in the case of 976-nm semiconductor laser, it is technologically difficult to completely
stabilize the spectrum under the influence of possiblemode hopping. Accordingly, we broadened the spectrumof the 976-nm semiconductor laser thereby optimizing it
to an extent such that a conversion efficiency resulting inan output power of not less than 20 mW be obtained andthat the influence of mode hopping of the laser on the
SHG output power be minimized. Figure 3 shows a typical spectrum of the fundamental lightwave thus determined.
In the case of a light source with a fundamental lightwave of broad line width as shown in Figure 3, it is known
that the sum frequency generation (SFG) theory works
better than the SHG theory in calculating the secondary
wave in good agreement with experimental results.
Let us, in accordance with the SFG theory, divide theincident lightwave into N fractions according to the wavelength and assume that a lightwave with an arbitrary
wavelength λi couples with all the lightwaves with a wavelength λj in an independent manner to generate a lightwave with a wavelength λij such that
1
1
1
=
+
λ ij
λi λ j(1).

Writing the phase mismatch condition as

SIMULATION AND FUNDAMENTAL
EXPERIMENTS

In SHG that takes advantage of non-linear optical effects,
both conversion efficiency and output power stability
Table 1∆ k = kij − ki − k j − kG(2)where: kG is the coefficient related with the polingreversal frequency of PPLN,
the SFG intensity can be obtained by solving the electricfield equations as

Target specifications.
2

Parameter

I s (ζ ) 2 =Unit

HPU50204 spec.

Power

mW

≥20

Wavelength

nm

488±2

Spatial mode

M2

≤1.2

Beam asymmetry

—

≤1.15

Beam diameter (1/e2)mm

0.7±0.15

Beam divergence

mrad

≤1.25

Pointing stability

µrad

≤10

Noise (20 Hz~2 MHz, rms)

%

≤0.25

Noise (20 Hz~20 kHz, p-p)

%

≤1

200

Long term power stability(2 h, ±3°C)

%

≤2

100

Warm up time

min

≤5

0

Polarization ratio

dB

≤16

Operating temperature°C

10 ~ 40πω ij W 2 2
1 N 8��
u ija sn ( uijb ζ , γ� )∑N i, j −1 c 2 kij

(3).

700
600
500
Power (mW)

4.

400
300

975.0

975.2

975.4

975.6

975.8

976.0

976.2

Wavelength (nm)Figure 3

Typical spectrum of 976-nm fundamental lightwave.

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Development of a 488-nm Blue Laser for Biomedical Analyzers

Here,c 2 kiui = 
 1/ 2 E ( λ� i)2�� i W109
8πω

and

c 2k juj = 
 1/ 2 E (�λ j )2
�� j W1098πω

to achieve good agreement between the calculated andexperimental results.
Figure 4 shows comparisons of spectrum between thecalculated and experimental results. When the controlledtemperature for the PPLN is changed, the phase mismatch condition shifts resulting in a change in the secondary harmonic spectrum. This, in addition to good
agreement between the calculated and experimental
results, can clearly be seen in Figure 4.
Figure 5 plots the conversion efficiency when the spectrum shown in Figure 3 is used as a fundamental lightwave. It has been confirmed that a conversion efficiency
of not less than 10 % over the fundamental lightwave, i.e.
40 mW or higher in terms of 488-nm output power, hasbeen achieved amply allowing the manufacture of lightsources with an output power of not less than 20 mW.

∆ k' = ∆ kα� = ( kij − ki − k j − kG )α�(4)c 2 kij

uij = 
 1/ 2 E ( λ� ij )
2
�� ij W109
8πω
2

2

2

� jω
� ij  1/ 2
8�π W 1/ 2 �ω i ωζ� = K  2  
 z
 c 
 ki k j kij 

and
kj
kij
c2
 ki

� i )2 +E( λ� j )2 +E( λ� ij ) 2
 E( λ
� i
� j
ω
� ij
ω
8π
� ⋅109  ω(5).

Equation (5) defines the lightwave propagation within the
waveguide.
In effect, however, all fundamental lightwaves interact
with each other, which should be taken into account in thederiving process of SFG. Accordingly, two variables f(N)and α are introduced here as

5.

CHARACTERISTICS EVALUATION

Figure 6 shows the appearance of the blue laser (productcode: HPU50204) developed here. The product has

W' = f ( N ) ⋅W

Nomenclaturec velocity of light
E electric field
f interaction function
I intensity
k phaseK effective nonlinear function of PPLN
N wavelength fraction
sn Jacobi elliptic functions
u substitution variable
W power flow per unit area parallel to the propagation direction
z direction of propagation
Greek letters
α interaction coefficientζ substitution variable
λ wavelengthω angular frequency
Subscripts
i fundamental index
j fundamental index
ij SFG index
G PPLN period
S SFG

Experimental(6)

Calculated∆ T= 0°C
SG intensity (mW/(m2nm))

 ki

kj
kij
c
� i )2 +E( λ� j )2 +E( λ� ij ) 2  E( λ8π
� ⋅109  ω
� i
� j
ω
� ij
ω


30

∆ T= 2.5°C

20∆ T= -2.5°C

10

0

487.8

Figure 4

487.9

488.0
488.1
Wavelength (nm)

488.2

488.3

Comparisons of spectrum between the calculated
and experimental results.

50

12Conversion efficiency

45
Output power of 488 nm (mW)

= f ( N)40
2

Output power of 488 nm

40
35

10
8

30
25

6

20
4

15
10

2

5
0

Conversion efficiency (%)

W=

(7)

0

Figure 5

100
200
300
Input power of 976 nm (mW)0400Conversion efficiency and output power using the
SFG theory.

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Development of a 488-nm Blue Laser for Biomedical Analyzers

25Power (mW)

20
15
10
5
0

0

30

60

90

120

300

400

Time (min)Figure 6(a) For 2 hours

Appearance of HPU50204, 488-nm SHG blue laser.
25

Table 2

5
0

0

100

200
Time (h)(b) For 2 weeks
Output power stability.

488.0

0.120

487.9

0.115

487.8

0.110

487.7

0.105

487.6

0.100

487.5

0.095

487.4

0.090

487.3

0.085

487.2

0.075
7201

6601

6001

5401

4801

4201

3601

3001

2401

1801

1201

487.0

0.080

Center (nm)
FWHM (nm)487.1

FWHM (nm)Figure 7

0.070

Time (s)Figure 8

Results of wavelength stability measurement.

Results of noise characteristics measurement.

Optical power
(mW)

20

10

1

5.3 Wavelength Stability
The stability of output spectrum was measured using aspectrum analyzer (ANDO AQ6315). The results areshown in Figure 8. Both the center wavelength and FWHM
are seen to be stable, indicating that the spectrum of fundamental lightwave is stabilized and that the temperatureof PPLN is stably controlled.

15

601

5.2 Output Power Stability
Output power stability was evaluated and the results areshown in Figure 7. The stability was 0.24 %, amply satisfying the specification of less than 2 %. In the continuous
operation for two weeks the stability was 1.37 %, a satisfactory value.

Power (mW)

5.1 Noise Characteristics
The output laser light was photoelectrically converted
using a photo diode, and after I-V conversion, fluctuation
components (p-p, and rms) at each frequency band wasmeasured using an oscilloscope. Table 2 shows theresults.
Whereas noise characteristics are important for lightsources for biomedical analysis, the laser has achieved
the target specifications of p-p < 1 % and rms < 0.25 %with a sufficient margin.

20

Center wavelength (nm)achieved a compact size of 177L × 127W × 40.8H and alow power consumption of 18 W. The use of a pigtail fiberfor output termination allows easy handling, and it can beadapted to application of connector or collimator.

Filter (kHz)Noise (%)
p-p

rms

20000

0.124

0.027

1280

0.144

0.032

320

0.210

0.039

160

0.185

0.035

20

0.210

0.034

10

0.210

0.036

5.4 Temperature Characteristics
Figure 9 shows the results of temperature characteristics
measurement. In the measurement, temperature cycle of
10°C~40°C was applied with a temperature gradient of
6°C/h, severer than the specification of 6°C/2h. The output
power levels at 10°C and 40°C were 20.38 mW and 19.85
mW respectively, indicating a fluctuation margin of 2.6 %,a satisfactory stability in the service environment.
5.5 Beam Property
The output beam of the blue laser developed here wascollimated using a f3.85 lens, and the beam parameters

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Development of a 488-nm Blue Laser for Biomedical Analyzers
50

25Collimator head

20

40

15

30

10

20

5
0

10

Power
Temperature cycle
0

2

4

6

8

10

12

Figure 11 Measurement system for beam pointing stability.

shows satisfactory stability below the detection limit of
0.0024 mrad.

0

Results of temperature characteristics measurement.

were measured using a beam profiler. The results areshown in Table 3 and Figure 10. It can be seen that an
ideal beam shape of M2 < 1.2 has been obtained, andthat both the beam width and beam divergence meet thetarget specification.
In this connection, because the output of the blue laser
developed here is terminated with a pigtail fiber, any
beam width is available by selecting an appropriate lens.
5.6 Beam Pointing Stability
Figure 11 shows the evaluation system for beam pointing
stability. While beam pointing stability poses a problem inargon lasers because their beam position fluctuates dueto heat generation and vibration, the blue laser developed
here has its pigtail fiber termination collimated, so that
there is no problems whatever in terms of heat generationand vibration of laser head. The measurement result
Table 3CCD

CRTTime (h)Figure 9

Auto collimator

HPU50204

Temperature (° C)

Power (mW)

PMF

6.

LONG-TERM RELIABILITY

The long-term reliability is now being evaluated undercontinuous operation. As of July XX, 2005, there is no
problems after Y months have elapsed. A long service life
is reasonably anticipated since a semiconductor laser fortelecommunications is used for fundamental lightwave
generation.

7.

CONCLUSION

The authors have developed the 488-nm blue laser by
combining the 976-nm semiconductor laser with thewavelength conversion device. Due to its simple device
configuration the laser has a long service life, compact
size, low cost, and low power consumption. In the futurestrong demand is expected as a light source for bioinstrumentation in place of argon laser and DPSS laser.

Results of beam parameter measurement.
Parameter

X

YR

Spatial mode

2

(M )

1.049

1.112

1.081

2Wo

(mm)

0.57220.59880.58592We(mm)

1.00151.0512

1.0266

Zo

(m)

-0.72

-0.7471-0.7337Zr(mm)

0.50120.51780.5099Divergence(mrad)

1.142

1.156

1.149

Astigmatism (Zoy/Zox)/Zrr(%)

REFERENCES
1) M. M. Fejer, G. A. Magel, D. H. Jundt and R. L. Byer: J. Photo Elect.
28 (1992) 2631.
2) J. A. Amstrong, N. Bloembergen, J. Ducuing and P. S. Pershan:
Phys. Rev. 127 (1962) 1918.
3) A.Yariv: Optical Electronics (Saunders College Publisher, City of
publication, 1991) 4th ed., Chap. 8, p. 258.

5.31

Waist asymmetry (2Woy/2Wox)

1.0465

Divergence asymmetry (θy/θx)

1.01295
X widthBeam width (mm)4

Y width
R width

3
2
1
0
-10

-5

0

5

Distance (m)Figure 10 Beam width in external propagation.

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