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Photonic Laser Propulsion (PLP):
Photon Propulsion Using an Active Resonant Optical Cavity
Young K. Bae*
Bae Institute, Tustin, California, 92780
We present an innovative Photonic Propulsion concept, Photonic Laser Propulsion (PLP),
which uses direct momentum transfer of photons for thrust generation, and exploits a novel
photon thrust amplification scheme that increases the thrust to power ratio by orders of
magnitudes. The amplification is accomplished by trapping or bouncing photons between
two high reflectance mirrors located separately in spacecraft platforms. With this enhanced
photon thrust the thrust to power ratio of PLP engines is competitive with the conventional
electric thrusters. PLP provides Isp = 3x107 sec, and is propellantless, thus theoretically, it
can accelerate spacecraft to the velocities orders of magnitude larger than conventional
rocket velocities. In addition, PLP can provide ultrahigh precision in thrust as well as thrust
vector pointing. This aspect permits the usage of PLP in precision control of spacecraft
clusters or precision docking of spacecraft. We have recently successfully demonstrated the
proof-of-concept of PLP in the sub-scale laboratory setup. The maximum photon thruster
achieved so far in this setup was 35 µN at the laser output of 1.7 W with the use of high
reflectance mirror with a 0.99967 reflectance, corresponding to an apparent photon thrust
amplification factor of ~3,000. The results on this demonstration are presented in a
concurrent session. The crucial aspect of the current innovation is in the usage of an active
resonant optical cavity in which the laser cavity is directly formed between a pair of space
platforms. During the PLP demonstration, we have discovered that the optical gain
medium can rapidly adapt to any changes in the optical cavity parameters by amplifying the
resonant photon waves at a given cavity condition, thus the cavity becomes robust against
changes in the cavity length or acceleration of cavity mirrors. The reason for this is that the
optical gain medium is within the optical cavity, and the cavity is operated in multifrequency
oscillation.
Nomenclature
c = light velocity, 3 x 108 m/sec
FT = photon thrust
g = the gravity acceleration constant, 9.8 m/sec2
h = the Plank constant
Isp = specific impulse
L = maximum acceleration distance
λ = photon wavelength
M = spacecraft mass
•
M
= photon mass flow rate
N = photon number flux
ν = photon frequency
P = extracavity laser output power
r = mirror diameter
R = output coupler mirror reflectance
S = apparent photon thrust amplification factor
*218 W. Main St., Suite 102, Tustin, CA 92780, Member AIAA
AIAA SPACE 2007 Conference & Exposition
18 - 20 September 2007, Long Beach, California AIAA 2007-6131
Copyright © 2007 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
Introduction
The propulsion concept, Photonic Propulsion, of using direct momentum transfer of photons to propel spacecraft
has been around since the beginning of the 20th century.1 According to Special Relativity, the highest velocity of the
rocket exhaust particle can have is the light velocity, c = 3 x108 m/sec. Therefore, photons are the ultimate rocket
fuel that will produce extremely high specific impulse, Isp. The Isp of Photonic Propulsion can be derived from the
following equations. The Isp of a rocket engine is given by:
•
=
Mg
F
IT
sp , (1)
where FT is the photon thrust, g is the gravity acceleration constant, and
•
M
is the mass flow rate. For Photonic
Propulsion, FT of photon flux is given by:
c
Nh
FT
ν
= , (2)
where N is the photon number flux, h is the Plank constant, and ν is the photon frequency. To be simplistic, here we
assume that all photons have a single frequency, ν. The mass flow of the photons is different from that of the non-
relativistic fuel exhaust particles, because the photon does not have a rest mass. However, in the relativistic sense,
the mass and energy are equivalent, and when the rocket emit photons, it loses small amount of mass through the
energy loss. Thus, according to the mass-energy equivalence principle, E = mc2, the equivalent mass flow
•
M
of
photons is given by:
2
c
Nh
M
ν
=
•
. (3)
By combining Eqs. 2 and 3 with Eq. 1, one obtains:
sec1006.3 7
×==
g
c
Isp . (4)
Although photons have the highest Isp, the specific thrust, defined here as the thrust to power ratio, of photonic
propulsion is many orders of magnitude smaller than conventional propulsion including electrical and beamed-
energy propulsion. The specific thrust of the photon propulsion, Fs is given by:
cNh
F
FT
s
1
==
ν
. (5)
Fig. 1 shows the thrust to power ratio, which is defined as specific thrust here, of electric thrusts that include Hall
thrusters and Pulsed Plasma Thrusters, and that of photon thrusters. The specific thrust of the photon thruster is
several orders of magnitude smaller than that of conventional electrical thrusters, because it has the highest Isp. The
green line in Fig. 1 represents 1/Isp curve that shows the general behavior of the specific thrusts of various electric
thrusters as well as that of the photon thruster. Therefore, the inefficiency in producing thrust at extremely high Isp,
is a universal tendency (the law of physics) in all thrusters, and it is not unique in the photon thruster. In other
words, if conventional electric thrusters can be made to have Isp~107 sec, theirs specific thrust would be similar to
that of photon thrusters.
*218 W. Main St., Suite 102, Tustin, CA 92780, Member AIAA
Isp (sec)
102103104105106107108
Specific Thrust (mN/W)
10-7
10-6
10-5
10-4
10-3
10-2
10-1
100
X 1,000
X 10,000
X 100
X 20,000
Electric Thrusters Photon Thrusters
Intracavity Multiplication Factors
Figure 1. Specific thrusts as functions of Isp of various conventional and photon thrusters.
To generate enough thrust to accelerate spacecraft, traditional Photon Propulsion concepts require extremely
high power beams of photons possibly generated from either the large solar collectors2 or nuclear power generators.3
These approaches require large scale and extremely heavy space structures, and their implementation may be
possibly many decades away or may not be economically viable. Antimatter based photon propulsion was also
considered,4 but the practical antimatter fuel production, storage, and engine were estimated to be 100 years away.
One way of making photon propulsion more viable to the near-term applications, is overcoming the inherent
inefficiency in producing thrust of the photon thruster by amplifying the momentum transfer of photons by bouncing
or trapping photons between two high reflectance mirrors that form an optical cavity. As shown in Fig. 1, if the
photon thrust amplification is more than 1,000, the specific thrust of the photon thruster starts to compete favorably
with the electric thrusters with much lower Isp. Although the photon propulsion based on this kind of multiple
reflections of laser beams is highly attractive, it turned out that the implementation of the concept is not
straightforward. Two types of optical cavities for amplification of the thrust of Photon Propulsion have been
around: 1) passive resonant optical cavities, 2) non-resonant optical cavities.
Passive Resonant Optical Cavity Schemes
A straightforward idea of bouncing or trapping photons can be demonstrated in a simple optical cavity with the
use of two high reflectance mirrors. A passive resonant cavity, in which the laser beam is injected into the cavity
without any amplifying medium, is a form of resonant optical cavities. The schematic diagram of passive resonant
cavity approach to photon thrust amplification is illustrated in Fig. 1. The passive resonant optical cavity is called
Fabry-Perrot optical resonator,5 and has been extensively used in high-sensitivity optical detection methods, such as
the cavity ring down spectroscopy. In the cavity ring down spectroscopy, typically laser pulses are injected through
the first mirror and bounced between two mirrors as many as tens of thousand times. The current off-the-shelf
technological limit of the system reported to date is obtained with super mirrors used for the cavity ring down
spectroscopy6 (currently available in the advanced research grade only) with the reflectance of 0.99995 with the
photon bounce number of 20,000.
*218 W. Main St., Suite 102, Tustin, CA 92780, Member AIAA
W
F = 2W
___
T c
W
___
T
T
F = 2W
___
T c
F = 2W
___
T c
F = 2W
___
T c
W
Figure 2. Schematic diagram illustrating passive resonant optical cavity (Fabry-Perrot) approach to photon
thrust amplification. Assuming a single-line laser beam input and perfect reflection of the second mirror, the
intracavity laser beam intensity is amplified by a factor of 1/T, where T is the transmittance of the first
mirror.
However, the passive resonant optical cavity for photon thrust amplification is not useful for propulsion
applications, because it is highly sensitive to the small changes in the distance5 between the mirrors and mirror
deterioration. This sensitivity was observed in the gravitational detection system with high passive optical Q
cavities, in which even one nanometer perturbation in cavity length sets the system out of resonance and nulls the
photon thrust.7 In addition, the passive resonant optical cavity requires near single-frequency lasers to efficiently
inject the laser through the input mirror. Typically such single-frequency lasers have poor power-to-photon
conversion efficiency. Therefore, it is concluded that the passive resonant cavity photon thruster is impractical for
Photon Propulsion.
Non-Resonant Optical Cavity Schemes
The disadvantages of the passive resonant optical cavity approach can be simply overcome by avoiding
resonance condition of photons in the cavity. This non-resonant optical cavity, such as Herriot cells, approach to
Photonic Propulsion has been proposed.8,9 The schematic diagram of the Herriot-cell approach is shown in Fig. 2.
This approach requires highly focused laser beam spots on each mirror to avoid the beam interference that may
result in optical resonance in the cavity
However, as the cavity length and the number of photon bouncing increase, the focal spot diameter projected on
mirrors increases, requiring extremely large mirrors to avoid the laser beam interferences. Once the laser beam
starts to interfere, the non-resonant cavity becomes a passive resonant cavity that is impractical for photon
propulsion. In addition, the photon propulsion based on non-resonant optical cavities greatly suffers from the
extreme difficulties in the laser alignment to avoid the beam interference.
An attempt of demonstrating photon thrust amplification in a non-resonant Herriot-cell type optical cavity was
performed by Grey et al.10 In their experiment, a small portion of a laser beam from a 300 W CW Nd:YAG laser
was injected into an optical cavity formed by two high reflectance dielectric mirrors. One of the mirrors was drilled
through to have a small hole to allow the injection of the laser beam. The photon thrust was measured with a
vacuum compatible microbalance. They obtained amplified photon thrust of ~0.4 µN and a photon thrust
amplification factor of ~2.6, which was much smaller than the predicted amplification factor greater than 50.10 The
much lower-than-expected amplification factor obtained by Grey et al.10 probably resulted from the above
mentioned technical difficulties in the non-resonant optical cavity concept. Therefore, the previously proposed
concepts based on multiple reflections in the non-resonant optical cavity of laser beams seem to be impractical, and
searches for a viable photon propulsion concept continues.
*218 W. Main St., Suite 102, Tustin, CA 92780, Member AIAA
W
F = W
___
C
W
F = 2W
___
CF = 2W
___
C
W
F =2NW
___
CF =2NW
___
C
Figure 3. Schematic diagram illustrating the Herriot-cell-type approach to photon thrust amplification. The
first figure shows the thrust on the first mirror without the second mirror. The second figure shows the laser
beam is bounced once resulting in the thrust of 2W/c. The third figure shows the laser beam is bounced N
times resulting in the thrust amplification by a factor of N. However, as the cavity length and the number of
bouncing increase, the reflected laser beams begin to overlap resulting in Fabry-Perrot-type behavior.
Photonic Laser Propulsion (PLP)
Photonic Laser Propulsion (PLP) is an innovative amplified Photonic Propulsion concept, which is based on the
active resonant optical cavity in which an amplification medium is located within the optical cavity. In PLP, a laser
cavity is formed between two space platforms with the laser gain media located between them as illustrated in Fig.
4, in contrast to the previously proposed multiple reflection laser photon propulsion concepts that use passive optical
cavities with the laser amplification located outside of optical cavity.
W
F = 2W
___
T c
W
___
T
T
F = 2W
___
F = 2W
___
T c
F = 2W
___
Gain
Medium
G
Figure 4. Schematic diagram of Photonic Laser Propulsion, which is based on the active resonant optical
cavity approach to photon thrust amplification.
In PLP, the active resonant optical cavity scheme overcomes the technological challenges of passive optical
cavity photon thrusters in propulsion applications. First, in PLP, there is no difficulty in injecting the power into the
cavity, because the laser beam is directly formed within the cavity. Secondly, but more importantly, PLP is
strategically operated in multifrequency-multimode, it is highly stable against the perturbations in cavity parameters,
such as the cavity length.
In PLP, a laser cavity is formed by two mirrors located separately in two spacecraft platforms, the photon thrust,
FT, produced by a laser beam on each mirror is given by:
*218 W. Main St., Suite 102, Tustin, CA 92780, Member AIAA
c
PRS
FT
2
=, (6)
where P the is extracavity laser output power through the output coupler mirror, R the output coupler mirror
reflectance ~1, and S is the apparent photon thrust amplification factor that is the ratio of the intracavity laser power
to the extracavity laser power, P. S is approximately given by:
R
S−
=
1
1 . (7)
The optimum design of the PLP thruster is different from that of the typical laser cavities. The cavity design of
the typical lasers is tailored to maximize the laser output power in the extracavity. Depending on the characteristics
of the gain media, the reflectance of the output mirror (output coupler) is chosen 0.9 – 0.99 for the conventional
laser cavities. To minimize the absorption loss in the gain media, the PLT should be designed to maximize the
intracavity power, thus the gain media should be very thin to minimize the absorption loss in the gain media, similar
to the one used in the state of the art solid state disk lasers used for intracavity second harmonic generation, except
without the need of the frequency doubling crystal. In PLP, thus the thermal management of the gain media
becomes an important issue. In reality, because of the limitation in the laser gain medium and other thermal effect,
the amplification fact should be consider given by Eq. 7 should be considered as upper bounds.
Static Applications of PLP
In this section, a small scale (mN capacity) PLP engine is defined as a Photonic Laser Thruster (PLT). Many in-
space applications exploiting PLT are currently envisioned. With the use of rapidly evolving high power laser
technologies, such as Diode Pumped Solid State (DPSS) Laser technologies, PLT can be compact, light, and energy
efficient, ideal for space borne applications. One of the most important static applications of PLT is in precision
spacecraft formation flying and distributed or fractionated spacecraft architectures. We have recently successfully
demonstrated the proof-of-concept of the photon thrust amplification in the sub-scale laboratory setup. The
maximum photon thruster achieved so far in this setup was 35 µN at the laser output of 1.7 W with the use of high
reflectance mirror with a 0.99967 reflectance, corresponding to an apparent photon thrust amplification factor of
~3,000. The details of this result are presented in the concurrent session of this conference.11 Therefore,
implementation of the static PLT applications became much more realistic.
Table 1 shows examples of maximum theoretical thrusts obtained by PLT at various extracavity laser powers for
static applications. For estimating the theoretical limit on maximum photon thrust, other parameters including
thermal limitation and optical absorption and saturation of the laser gain media are neglected, and results of the
maximum theoretical thrusts as a function of the reflectance of the mirrors at an extracavity laser power of 10 W are
summarized in Table 1.
TABLE 1.
Maximum Operation Laser
Power (extracavity)
HR Mirror Reflectance Maximum Photon Thrust
10 W 0.9999 (research grade) 670 µN
10 W 0.99995 (typically used super mirror) 1.34 mN
10 W 0.9999998 (with 1 ppm mirror loss) 22.3 mN
One primary application of PLT is in a nano-meter accuracy formation flight method with photon thrusters and
tethers, Photon Tether Formation Flight (PTFF), with the maximum baseline distance over 10 km for next
generation space applications.12,13 PTFF is stabilized by PLTs and tethers, thus it is contamination-free and highly
power efficient, and provides ample mass savings. In addition, PTFF is predicted to be able to provide an
unprecedented angular scanning accuracy of 0.1 micro-arcsec, and the retargeting slewing accuracy better than 1
micro-arcsec for a 1 km baseline formation.12,13 Another important emerging application of PTFF is in distributed
and fractionated spacecraft architecture, such as F-6 System architecture.14,15 Because PLT can be adapted to all-in-
one system for the interferometric ranging, navigation sharing, power sharing, optical communication, force and
torque sharing;14,15 PTFF can minimize the system overhead and necessary service function duplication, the cost,
*218 W. Main St., Suite 102, Tustin, CA 92780, Member AIAA
weight and power consumption. In addition, PTFF does not require propellant, thus provides significant propulsion
system mass savings, and is free from propellant exhaust contamination, ideal for signature-free missions with
highly sensitive sensors. Therefore, PTFF seems to be an excellent system approach to distributed and fractionated
space architectures. The details of this are presented in the concurrent session.16
Dynamic Applications of PLP
PLP can be used for dynamic in-space applications, such as precision docking, orbit changing and spacecraft
acceleration to ultrahigh velocities. For precision spacecraft docking and minor orbit changing, the required thrust
would be in the order of 1 N. Table 2 shows examples of maximum theoretical thrusts obtained by PLP engines at
various extracavity laser powers for dynamic applications. For estimating the theoretical limit on maximum photon
thrust, other parameters including thermal limitation and optical absorption and saturation of the laser gain media are
neglected, and results of the maximum theoretical thrusts as a function of the reflectance of the mirrors at an
extracavity laser power of 1 kW are summarized in Table 2.
TABLE 2.
Maximum Operation Laser
Power (extracavity)
HR Mirror Reflectance Maximum Photon Thrust
1 kW 0.9999 (research grade) 0.670 N
1 kW 0.99995 (typically used super mirror) 1.34 N
1 kW 0.9999998 (with 1 ppm mirror loss) 22.3 N
In this scenario, the parent spacecraft platform would carry 1 kW PLP engine, and the daughter spacecraft
platform the HR mirror system. The deceleration can be performed by forming the PLP optical cavity at a
determined distance.
Table 3 shows examples of maximum theoretical thrusts obtained by PLP engines at various extracavity laser
powers for spacecraft acceleration applications. For estimating the theoretical limit on maximum intracavity laser
power and the corresponding thrust, other parameters including thermal limitation and optical absorption and
saturation of the laser gain media are neglected, and results of the maximum theoretical thrusts as a function of the
reflectance of the mirrors at an extracavity laser power of 10 MW are summarized in Table 3.
TABLE 3.
Maximum Operation Laser
Power (extracavity)
HR Mirror Reflectance Maximum Theoretical Thrust
10 MW 0.9999 (research grade) 670 N
10 MW 0.99995 (typically used super mirror) 1.34 kN
10 MW 0.9999998 (with 1 ppm mirror loss) 22.3 kN
When the mass of the launching system is much greater than the mass of the spacecraft, the spacecraft maximum
velocity, Vmax, with PLP, which is much smaller than c, is given by:
M
LF
VT
2
max = (8)
where L is the distance of the acceleration and M is the mass of the launched spacecraft. For example, if the
scattering and absorption of the optical systems are negligible, with 10 MW laser system, 0.99995 reflectance
mirrors, M = 1 kg, and L=1,000 km, Vmax > 51.8 km/sec. The same system with 0.999998 reflectance mirrors will
have Vmax > 200 km/sec. PLP can be used for deep-space rapid turn-around probing missions, which does not
require deceleration. Under this velocity the PLP spacecraft will transit the 100 million km to Mars in less than 6
days. The acceleration time is only ~10 sec, which is the firing time of the laser system. If we assume 10 % laser
efficiency, the total energy required for the laser without considering cooling system power consumption is 1 GJ.
Therefore, this PLP can be used for rapid outcome deep space missions. PLP can be scaled up for much larger
*218 W. Main St., Suite 102, Tustin, CA 92780, Member AIAA
spacecraft. If the mission requires breaking of the spacecraft, it is desirable to have the second PLP system with
reversed thrust for breaking or deceleration.
The maximum range of the acceleration operation in such a PLP system depends mainly on the diameter of
mirrors. The theoretical limit of the intracavity length is ~ L, for a confocal cavity resonator is given by17
λ
21rr
L= (9)
where r1 and r2 are the radii of the laser beam projected on the mirrors, and λ is the wavelength of the laser. For
example, λ = 10-6 m, L= 1,000 km, if the mirror radius of the launched spacecraft is 0.2 m, the required minimum
radius of the launching system mirror is 5 m. For L = 1,000 km system, various numerical examples of the mirror
radius requirements are summarized in Table 4. Table 4
Launched System Mirror Radius Required Launching System Mirror Radius
1 m 1 m
0.5 m 2 m
0.2 m 5 m
0.1 m 10 m
The diameters and weights of these mirrors have to be chosen strategically depending on mission characteristics.
For small spacecraft with a weight in the order of 1 kg, probably the maximum allowable diameter of mirror due to
the weight limitation would be ~ 0.1 to 0.2 m, requiring the launching system mirror diameter to be 5-10 m. The
fabrication of the high quality super mirrors with radii in the order of 1 m is well within the currently available state-
of-the-art mirror manufacturing technologies, and based on the current space telescope mirror technology, the
availability of super mirrors with diameters of 5 – 10 m seems likely to happen in the near future. Therefore, the
present invention can be used to maintain the intersatellite distances of ~1,000 km. However, if the large diameter
mirrors can be fabricated and carried in the larger satellite platforms, there is no physical limit on the intersatellite
distance.
Effect of Doppler Shift
One of the factors that limit the maximum obtainable velocity of the accelerating mirror and its accommodating
spacecraft is limited by the Doppler shift of the bouncing photons. Doppler shift effect on the active resonant cavity
behavior is an extremely complicated issue, which is beyond the scope of the current paper. Eventually, this aspect
should be studied with computer optical simulation. Optical gain in the laser cavity can only occur for a finite range
of optical frequencies. The gain bandwidth is basically the width of this frequency range. For example, the gain
bandwidth of the YAG laser system with the laser wavelength in the order of 1,000 nm is in the order of 0.6 nm,17
which is ~ 0.06 % of the wavelength. For an order of magnitude estimation, we assume that PLP utilizing the YAG
laser system will be limited by the gain bandwidth to the first order, then, theoretical maximum spacecraft velocity
is ~1.8 x 105 m/sec (180 km/sec) that is 0.06 % of the light velocity, c=3x108 m/sec. We are currently investigating
this Doppler shift effect on PLP limitations in accelerating spacecraft.
Acknowledgments
This work was supported by Internal Research Fund of Bae Institute.
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*218 W. Main St., Suite 102, Tustin, CA 92780, Member AIAA
